1. Introduction
The broad range of Hermann von Helmholtz's (1821–1894) scientific interests, as well as his numerous contributions to the development of nineteenth-century science, have never ceased to amaze scholars in the exact sciences, as well as philosophers. Helmholtz's pivotal role in the instrumental and laboratory revolution in the nineteenth century has become common knowledge: not only is he widely recognized as “a leading figure in shaping philosophy of science during the second half of the nineteenth century” (Heidelberger Reference Heidelberger and Cahan1993, 461), he is furthermore credited as one of “the most innovative experimentalists of his day” (Finger and Wade Reference Finger and Wade2002a, 137), and even as “one of the last great figures of the Aufklärung” (Cahan Reference Cahan1993, 559).
The revolutionary character of Helmholtz's scientific attitude can be highlighted by turning to a lecture the scientist gave in 1877, at the age of 56, in which he looked back at the conditions under which he once studied medicine at the Friedrich-Wilhelm Institute in Berlin from 1838 to 1842:Footnote 1
The medical education of that time was based mainly on the study of books; there were still lectures, which were restricted to mere dictation; . . . there were no physiological and physical laboratories. . . . Any of my fellow-students who wished to make experiments had to do so at the cost of his pocket-money. . . . We had, it is true, an almost uncultivated field before us, in which almost every stroke of the spade might produce remunerative results. (Helmholtz [1877] Reference Helmholtz and Cahan1995, 319)
With respect to the development and popularization of the experimental method in science, Helmholtz is to be credited with more than one “stroke of a spade.” In the decade after obtaining his doctoral degree in medicine at the Friedrich-Wilhelm Institute in Berlin in 1842, Helmholtz performed groundbreaking experiments on fermentation and putrefaction, heat production, muscular contraction, and nerve conduction, and published his seminal paper On the Conservation of Force [Über die Erhaltung der Kraft] ([1847] 1889).Footnote 2 Through these investigations, Helmholtz gathered a body of experimental evidence that supported a physico-chemical approach to living processes, and thus contributed to making the metaphysical concept of life force [Lebenskraft] superfluous (Koenigsberger Reference Koenigsberger1902/03; Meulders Reference Meulders2010; Finger and Wade Reference Finger and Wade2002a). Thanks to these experiments, Helmholtz not only became a renowned member of the Berlin Physical Society [Berliner Physikalische Gesellschaft] (BPS), he was likewise appointed a professorship in Berlin (1848–1849), and later in Königsberg (1849–1855), where he embarked upon an area of research that he would continue until his death in 1894, i.e. the study of the psychophysiological nature of human perception (Koenigsberger Reference Koenigsberger1902/03; Helmholtz [1891] Reference Helmholtz and Cahan1995).Footnote 3
Within the scope of this paper, we are especially interested in Helmholtz's attempt to address the question of what it is to see at least from the early 1850s onwards, an endeavor that culminated in his three-volume Treatise on Physiological Optics [Handbuch der Physiologischen Optik] (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25).Footnote 4 In the Preface of his magnum opus, Helmholtz explains that the Handbuch aims in the first place at bringing order and coherence to the wide, disparate field of the study of perception, and in accordance with this goal, the work is primarily encyclopedic in character (ibid. 1, viii). However, the Treatise especially earned Helmholtz credit as the architect of the scientific study of human perception, because it introduced a number of new instruments and methods that allow for the objective measurement of physiological states, presented minute observations of physical structures, and furthermore comprised a proto-psychological analysis of the process of objectification. Hence, the work stood out particularly for its attempt at giving a comprehensive, all-encompassing account of perception, with its successive volumes subsequently going into “the physical characters of the eye, . . . the physiological processes of excitation and conduction . . . and lastly . . . the psychological question, how mental apprehensions are produced by the changes which take place in the optic nerve” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 128).
Interestingly enough, finishing up the third, psychological, part of his opus magnum seemed to be a real ordeal, at least if we go by Helmholtz's private correspondence with his close friend Emil du Bois-Reymond during the 1860s. From these it is clear that the scientist was really puzzled over the philosophical questions he was confronted with in attempting to articulate his psychological theory, as he was faced with issues that could not be answered on a strictly factual basis, and required him to “persuade people” with “the most superior arguments” (Helmholtz, as quoted in Turner Reference Turner1994, 74). In the third part of his Treatise, Helmholtz could no longer rely solely on facts derived from his experimental practice, and instead had to resort to philosophical arguments to defend and formulate the psychological dimension of his theory of perception (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 1: 1).
One of the distinctive features of Helmholtz's philosophical justification for the third volume, that has been keenly discussed ever since the nineteenth century, is his so called “dovetailing” between empiricism and (transcendental) idealism, as Hamner (Reference Hamner2003) calls it, or his attempt to reconcile both (Westheimer Reference Westheimer2008).Footnote 5 While Helmholtz is frequently identified as one of the earliest representatives of the neo-Kantian movement in Germany, he was as much a defender of methodological empiricism, claiming amongst others that “no other method is possible than that of endeavoring to arrive at the laws of facts by observations; and we can only learn them by induction” (Helmholtz [1877] Reference Helmholtz and Cahan1995, 322).Footnote 6 This peculiar state of affairs has inevitably led to very different readings of his work. While some emphasize its empirical dimension, others interpret it mainly against the background of German idealism, while still others have a more mixed reading and rather subscribe to the “dovetailing” or “attempt to compromise” hypothesis.Footnote 7
This paper aims at contributing to the ongoing efforts to get a firmer grasp of the systematic significance of Helmholtz's so-called “dovetailing.” However, contrary to existing analyses that commonly focus on Helmholtz's epistemological stance or his philosophy of science, the present exposition takes Helmholtz's proto-psychological work as a point of departure, and more particularly, his psychological problematization of the perceptual object.Footnote 8 To be precise, this paper aims at an in-depth analysis of the nature of Helmholtz's combined adherence to both idealism and empiricism in the context of his treatment of a problem that lies at the heart of his psychology of perception, i.e. the psychogenesis of the object in perception, or the question as to how the idea of “a thing” is generated from a mere “state of excitation” or “functional activity” in the nervous fibers (see, especially Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25); [1855] Reference Helmholtz1896; [1868, 1878, 1892] 1995). In addressing this question, both the negative and positive dimensions of Helmholtz's psychological theory will be considered, i.e. an analysis is presented of (i) the scientist's motive for establishing the problem of objectification as a psychological problem, and (ii) his subsequent attempt to address the issue in his theory of perception. The specifically psychological angle taken in this investigation not only prompts us to consider figures who have hitherto been treated as having only minor import for Helmholtz's interpretation (most importantly J.S. Mill and J.G. Fichte), it furthermore sheds new light on some central tenets of Helmholtz's psychological stance, that have hitherto remained underappreciated. For one thing, this analysis reveals an explicit voluntarist tendency in Helmholtz's psychological theory, and as such, it not only restores the continuity with other early psychological theories – most notably Wilhelm Wundt's voluntarist psychology – but likewise offers an alternative perspective on Helmholtz's notorious revision of Kant's transcendental theory of space.
The remainder of this paper is structured as follows. Section 2 presents an analysis of Helmholtz's so-called “dovetailing” in the construction of the problem of perception, against the background of (i) the scientist's own physical and physiological investigations, as well as his adoption and further expansion of Johannes Müller's Law of Specific Nerve Energies (section 2.1), and (ii) his defense of methodological empiricism in psychology (section 2.2). In Section 3, Helmholtz's psychological account of the object is outlined, and especially the way in which it combines the empiricist idea of associative construction with the transcendental concept of a priori constitution. To illustrate this multi-layered nature of Helmholtz's psychological approach, the programmatic similarity of Helmholtz's project with John Stuart Mill's 1865 Psychological Theory of the Belief in an External World is analyzed (section 3.1.) as well as the Kantian and Fichtean strands of his psychological project (section 3.2). In conclusion, it is argued that the distinctive feature of Helmholtz's work in comparison with strict methodological empiricism lies not so much in the latter's adoption of particular elements of (transcendental) idealism, but rather in the “thick” conception of epistemic subjectivity that is implied in his psychological theory, i.e. in his attempt to found his empirical theory of perception as an associative construction in a second-order constitutive account of what the mind or subject must be like in order to associate its way into the external world.
2. Helmholtz's Problematization of the Perceptual Object
To gain insight into Helmholtz's psychological approach to the problem of the perceptual object, two things should be considered, namely (i) the physiological roots of his epistemology, and (ii) his defense of methodological empiricism with regard to the analysis of mental phenomena. Both played a crucial role in Helmholtz's denial of the self-evident, immediate nature of the idea of externality in perception, and as such, motivated his psychological approach to the genesis of the object as presented in the third part of his Treatise. In section 2.1., the background to Helmholtz's assumption of a radical “non-isomorphism between the physical and the sensory” realm (McDonald Reference McDonald2003, 182) is outlined. In this context, Helmholtz's physical and physiological research, as well as his further expansion and Kantian interpretation of Johannes Müller's Law of Specific Nerve Energies [Gesetz der spezifischen Sinnesenergien] (Müller Reference Müller1826) should be considered, as they formed the basis of Helmholtz's epistemological views with regard to the subject-object relation in perception. However, Helmholtz's insight into the discontinuity between subjective states of excitations and the representation of an object, was not in itself enough to motivate his psychological approach (see section 2.2). It is only in combination with his empiricist methodology that the scientist arrives at a non-reductionist view of objectification “an act of apprehension” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 127). As is generally known, Helmholtz mainly articulated his empiricism in opposition to nativist or physiologically reductionist theories, which claim that “complete ideas of objects are produced by . . . organic mechanisms” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 357). Rather than focusing on the empiricism-nativism debate, however, the programmatic aspects of Helmholtz's defence of psychological empiricism will be discussed, and especially the way in which they resonate with John Stuart Mill's philosophical arguments for a psychological (as opposed to a metaphysical or intuitionist) approach to the mind.
2.1. Helmholtz's Physiological Epistemology
First and foremost, the motive behind Helmholtz's psychological problematization of the perceptual object derives from his physical and physiological research on the sensory apparatus. After his experimental studies on muscle action and nerve conduction in the 1840s and early 1850s, Helmholtz turned to the investigation of the physical structure and physiological functioning of the sensory apparatus. In accordance with this evolution in his research interest, Helmholtz's scientific practice was no longer solely dominated by “wet lab” science, but shifted towards the observational study of the nature and functioning of the sensory apparatus and the perceptual process (Finger and Wade Reference Finger and Wade2002b, 234). Through this research, Helmholtz gained thorough insight into what we might call the natural defectiveness of both the anatomical and physiological structure and functioning of the perceptual system. In his Treatise, Helmholtz not only compiled a vast amount of existing research in this respect – whether or not confirmed through his own experimental practice – his invention of numerous measurement instruments furthermore allowed him to open up new areas of research, and expand the possibilities of empirical research in this area dramatically.Footnote 9
For one thing, Helmholtz famously invented the ophthalmoscope – rather coincidentally, as he would later recall in his Autobiographical Sketch (Helmholtz [1891] Reference Helmholtz and Cahan1995, 387) – when he was preparing his lectures as a university professor in physiology in Königsberg. The young scientist first presented his invention to the audience of the Berlin Physical Society in 1850, before describing it in detail in a paper published a year later (Koenigsberger Reference Koenigsberger1902/03; Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 1; [1892] Reference Helmholtz and Cahan1995). Thanks to this instrument, Helmholtz could study the inside of a living eye, and carefully examine the blind spot, for example, as well as the numerous blood vessels that caused gaps or distortions in the visual field. In the same vein, Helmholtz's invention of the ophthalmometer in the same year, enabled him to measure the precise curvature of the cornea, and demonstrate, amongst other things, that the latter “is not a perfectly symmetrical curve, but . . . bent in various directions,” causing refractive error and a degree of astigmatism, even in healthy individuals (Helmholtz [1868] Reference Helmholtz and Cahan1995, 140).Footnote 10 Furthermore, Helmholtz described dark spots within the eye, irregularities in the structure and surface of the lens, and finally concluded that “the eye has every possible defect that can be found in an optical instrument,” and cannot in itself account for what it is to see (ibid., 147).
More importantly however, Helmholtz became convinced of the psychological character of the perceptual process through his appropriation and expansion of Johannes Müller's – Helmholtz's teacher of physiology in Berlin, and supervisor of his doctoral dissertation – epoch-making Law of Specific Nerve Energies (LoSNE). Although Müller had sketched the general outlines of LoSNE as early as 1826 (at the age of 25), in his On the Comparative Physiology of Vision in Men and Animals [Zur vergleichenden Physiologie des Gesichtssinns], the mature statement of it is to be found in the 1833/1840 Elements of Physiology [Handbuch der Physiologie des Menschen]:Footnote 11
That which through the medium of our senses is actually perceived by the sensorium, is indeed merely a property or change of condition of our nerves; . . . peculiar sensations of the nerves perceived by the sensorium are excited as frequently by internal as by external causes, . . . the feeling . . . is a condition of the nerves, and not a property of the things which excite it. (Müller [Reference Müller and Baly1833/40] 1843, 707)
As Crary (Reference Crary1992) aptly described it, Müller's law – which caused nothing less than an epistemological scandal, according to him – led to a complete blurring of the external-internal distinction in perception, or a dismantling of the “referential illusion.” That is to say, LoSNE basically stated the case for what we might call sensory underdetermination, i.e. the fundamentally underdetermined nature of sensation with respect to its cause, and as such, for a radical fissure of subject and object in perception. According to sensory underdetermination, all that is given in perception is a world of underdetermined nervous energies – qualities determined by the specific sense organ involved – that do not inherently refer to anything beyond themselves. Furthermore, it should be noted that this changed conception of the notion of “sensation” correlated with a modified understanding of the concept of “stimulus,” which has now become a mere generic term referring to “any agent, external or internal, capable of producing an effect on a nerve,” as Cassedy described it, so that “sensation can arise in the absence of truth” and “can be produced by events other than our encounters with external objects or states of affairs” (Cassedy Reference Cassedy2008, 424).
Helmholtz's research into sensory physiology led him to further expand Müller's insights, in two different ways. Through his work on color vision in the early 1850s, he became convinced that his contemporary Thomas Young's trichromatic theory of color provided a “further extension of Johannes Müller's law” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 161). In this theory, Young famously hypothesized that the specificity of color sensations must be ascribed to the specific reactivity of retinal receptors to different wavelengths of light. He distinguished three such receptors, respectively sensitive to the wavelength associated with red, green, and violet (ibid.). Through his experimental work on color mixing, and his observation of subjects suffering from color blindness, Helmholtz provided further empirical evidence for Young's theory (Koenigsberger Reference Koenigsberger1902/03; Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 2). In contrast to the latter, however, Helmholtz did not speak of different “receptors,” but rather of different nerve fibers, and furthermore hypothesized that while each of these fibers has its own wavelength specificity, it would still react faintly when stimulated by the other two (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 2:143; Finger and Wade Reference Finger and Wade2002b). This theory of color – which assumed that color vision emerges from three different (although partially overlapping) nerve sensitivities in the retina – would later become known as the Young-Helmholtz theory, and besides giving a good explanation for color blindness and anticipating the modern cone theory of vision, it contained a further extension of Müller's most fundamental insight, according to Helmholtz:
Just as the difference of sensation of light and warmth depends demonstrably upon whether the rays of the sun fall upon nerves of sight or nerves of feeling, so it is supposed in Young's hypothesis that the difference of sensation of colours depends simply upon whether one or the other kind of nervous fibers are more strongly affected. (Helmholtz [1868] Reference Helmholtz and Cahan1995, 161)
Secondly, Helmholtz developed an analogous hypothesis in the field of acoustics, by means of his experimental research into the origin of tonal specificity, published in his 1863 On the Sensations of Tone. In the latter work, the scientist outlined the results of his experimental research on the sensation of tones, which demonstrated convincingly that individual frequencies are associated with the specific receptivity of the nerve fibers in the cochlea (Helmholtz [1863] Reference Helmholtz and Ellis1912, 148).Footnote 12 In this sense, these sensory elements – which Helmholtz identified most likely as the arches of Corti – would behave much like piano strings, with each particular element resonating in response to a specific tone (Helmholtz [1863] Reference Helmholtz and Ellis1912; Meulders Reference Meulders2010, 170). From this research, Helmholtz concluded that:
The sensation of different pitch would . . . be a sensation in different nerve fibers. The sensation of a quality of tone would depend upon the power of a given compound tone to set in vibration . . . different groups of nerve fibres. . . . This is a step similar to that taken in a wider field by Johannes Müller. . . . The qualitative difference of pitch and quality of tone is reduced to a difference in the fibres of the nerves receiving the sensation. (Helmholtz [1863] Reference Helmholtz and Ellis1912, 148)
In contrast to his teacher, who relied heavily on rudimentary self-experimentation and self-observation, Helmholtz took LoSNE into the lab, so to speak, and in doing so, succeeded in extending Müller's case for sensory specificity, by showing there is not only a modal difference in sensory qualities – i.e. a difference across sensory systems – as Müller argued, but likewise a qualitative one, that applies to the sensations produced within a single sensory system (Helmholtz [1868, 1878] 1995).
If anything, however, Helmholtz's further expansion of Müller's law further deepened the enigma LoSNE created for the theory of perception, namely the question as to how “from the event . . . of neutral stimulation, . . . one concludes to the existence of an objective reality external to our own subjectivity” (Schmitz Reference Schmitz1995, 49). “How they [our minds] manage this construction,” Rachlin adds, “became the business of all of psychology for the next hundred years” (Rachlin Reference Rachlin2005, 43). Consequently, the latter credits Müller for establishing the basic problem for perceptual psychology.
In addition to his radicalization of LoSNE through his own research, Helmholtz established the epistemological consequences of Müller's (now expanded) insights with regard to sensory specificity as the foundation of the sign-theory of sensation. What LoSNE proves, Helmholtz argued, is that there is no “pre-ordained harmony of the inner and the external world” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 220). Helmholtz's theory of perception therefore accepts the thesis that “the nature of sensation depends primarily on the peculiar characteristics of the nervous mechanism; the characteristics of the perceived object being only a secondary consideration” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 2:4). Hence, he conceptualized sensation as a sign or symbol for (as opposed to an image of) external objects, i.e., as no more than a “changed condition of the nervous fibers which we call the state of excitation or functional activity,” devoid of any intrinsic similarity with “the agent inducing it” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 149).Footnote 13 In the end, Helmholtz concluded, “representation and that which is represented [Vorstellung und Vorgestelltes], . . . belong to . . . entirely different worlds, which no more admit of being compared with each other . . . than the letters of a book and the sound of the words they denote” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:19).
Furthermore, Helmholtz was one of the first to give Müller's Law its Kantian interpretation, and is therefore credited by some as one of the earliest figures of so-called physiological neo-Kantianism, i.e., the early neo-Kantian movement – with Helmholtz, Otto Liebmann, and Friedrich A. Lange as its main protagonists – that developed from the physiological interpretation to Kant's a priori.Footnote 14 As early as 1855, Helmholtz claimed that Müller's insights signified:
one of the most significant advances in sense physiology in recent times. According to it, the quality of our sensations . . . does not depend upon the perceived external objects, but on the sensory nerves which mediate sensation. . . . Just the same what the physiology of the senses has proven in recent times, Kant sought to prove earlier . . ., by pointing out the share that our . . . mental organization [Organisation des Geistes] has in the formation of ideas [Vorstellungen]. (Helmholtz [1855] Reference Helmholtz1896, 98–99)
Later on in his career, Helmholtz maintained that Müller's law is “an empirical statement of Kant's theoretical exposition with regard to the nature of the human faculty of knowledge” and “presented and made evident the nature and meaning of . . . the subjective form of sensation” (Helmholtz Reference Helmholtz1896, 249). In other words: Helmholtz argued that just as Kant had claimed that every possible experience is necessarily conditioned by the a priori forms of intuition and the categories of thought, Müller had shown that the fundamental material of perception (sensation) is conditioned and determined by our physiological make-up. In this way, LoSNE was an occasion for Helmholtz to call attention to the fundamentally conditioned nature of human experience and knowledge, and to reinstate the critical boundary between the subject and the mind-independent thing.
While LoSNE might be said to state the basic problem for the psychology of perception, Müller's nativism (or physiological reductionism), however, prevented him from developing a full blown psychological account of the perceptual process. For one thing, Müller argued that the general idea of an external object is acquired prenatally as an “obscure” representation:
The first obscure idea excited could be no other than that of a sentient passive “self” in contradistinction to something acting upon it. The uterus, which compels the child to assume a determined position, and gives rise to sensations in itself, is also the means of exciting in the sensorium of the child the consciousness of something thus distinct from itself and external to it. In this way is gained the idea of an external world as the cause of sensations. (Müller [Reference Müller and Baly1833/40] 1843, 717)
Consequently, “the certainty of the cogito, for Müller, is the certainty of the body sensing itself [sich selbst empfindenden Körpers]” as Heidelberger (Reference Heidelberger and Sandkühler1997, 41) described it. Furthermore, Helmholtz's teacher hypothesized that the spatial features of external perception originate from the spatial distribution of the nerve endings in the sensory system itself (Müller Reference Müller1826, 40; idem [1833/40] 1843, 716–717). Indeed, as Müller confirmed, his theory of perception amounts to the peculiar conclusion that the direct perceptual object is actually the retina itself.Footnote 15 In the end, Müller thus escaped the problems created by his LoSNE with regard to the subject-object transition in perception, by resorting to innate mechanisms, by virtue of which a dynamical and geometrical opposition between the self and the not-self is given prior to experience (Schmitz Reference Schmitz1995). Müller's nativism therefore amounted to the following conclusion:
As the doctrine of specific nerve energies establishes the essential subjectivity of all sensations, one could believe, that the question now becomes where these mere subjective experiences derive their objective [gegenständlich-objectiver] nature from. Only, this question does not at all exist for Müller. According to him, sensations are originally [von Haus aus] endowed with a certain degree of objectivity, notwithstanding their complete subjectivity. (Post Reference Post1905, 54)
This contrasts sharply with Helmholtz's methodological empiricism, which compelled him to establish an autonomous, non-reducible psychological level of analysis in the theory of perception.
2.2. Helmholtz's Methodological Empiricism
In sharp contrast to his teacher (and to his contemporary Ewald Hering), Helmholtz vehemently opposed nativist theories of perception, and instead emphasized the autonomy of psychological investigation vis à vis physiology (on this topic see especially Turner Reference Turner1994). Hence, his work can be understood against the background of the gradual emancipation of psychology in the nineteenth century, as it was finally established as an autonomous discipline with Wilhelm Wundt's – yet another student of Johannes Müller, and Helmholtz's assistant in Heidelberg from 1858 to 1864 – instauration of the Psychologisches Institut in Leipzig in 1879 (Rieber and Robinson Reference Rieber and Robinson2001). A decisive factor in this evolution was the emergence of what Hatfield called (non-materialistic) methodological empiricism or naturalism with regard to the mind, i.e., “the attempt to discover ‘natural’ laws of the mind, where ‘natural’ is cashed out through an analogy with the methods and modes of explanation in natural science, instead of by an appeal to ontology” (Hatfield Reference Hatfield1990, 17). Wundt's methodological framework was heavily influenced by John Stuart Mill's A System of Logic, which offered a model for scientific practice and reasoning inspired by chemist Justus von Liebig's scientific practice, amongst others (Schmidgen Reference Schmidgen2003; Richards Reference Richards, Bringman and Tweney1980). More particularly, Wundt claimed the latter work inspired his formulation of the psychological method in terms of the “complete decomposition [Zergliederung] of conscious phenomena into their elements” (Wundt 1882, 399, as quoted in Schmidgen Reference Schmidgen2003, 471). Furthermore, both Helmholtz's and the early Wundt's theories of perception as unconscious inference have demonstrable roots in Mill's theorizing (Richards Reference Richards, Bringman and Tweney1980).Footnote 16
Although John Stuart Mill has never written a treatise exclusively on the subject of psychology, he is frequently mentioned as one of the forerunners and inspirers of the development of empirical psychology in the second half of the nineteenth century in Germany (see, for example, Robinson Reference Robinson1995; Schmidgen Reference Schmidgen2003; Allik and Konstable Reference Allik and Konstabel2005). In his System, Mill outlined a view on scientific methodology and reasoning – “not the science of Belief, but the science of Proof, or Evidence”– based solely upon experimentation and experience, within an inductive framework (Mill [1843] Reference Mill1882, 21). Furthermore, he explicitly articulated his view in opposition to German, or a priori modes of thought, and common-sense intuitionism.Footnote 17
From his Treatise, it is clear that Helmholtz had read Mill's Logic, and based certain elements of his own theory on Mill's account of induction (see section 3.1.). Moreover, one cannot fail to notice a programmatic similarity in the rhetorical strategies used by both authors in defense of an autonomous psychological project. In addition to his rejection of a priori philosophy and intuition, Mill argued against ‘biologized’ psychology – or what we might call materialistic naturalism with regard to the mind – and rejected the hypothesis that mental phenomena are “generated through the intervention of material mechanisms” (Mill [1843] Reference Mill1882, 590). Instead, the English philosopher maintained that “there is a distinct and separate Science of Mind” (ibid., 591).Footnote 18 It is well known that for Helmholtz, the great dividing line in theories of perception ran between nativist and empiricist approaches, and indeed, the “dramatic juxtaposition of empiricism and nativism” was one of Helmholtz's main rhetorical strategies (Turner Reference Turner1993, 191). As such, both Helmholtz and Mill were later praised in Lange's History of Materialism for asserting “the rights of psychology . . . against the strictly materialistic view” (Lange Reference Lange and Thomas1881, 189).
While common-sense intuitionism can give no other proof for the direct givenness of the object (or the idea of the external) in perception than the intuitive “impossibility of getting rid of it,” as Mill called it, the nativist (or reductionist) approach is considered by both as audacious and insufficiently warranted (Mill Reference Mill and Mill1865, 187). The empirical method (as opposed to intuitionism and physiological reductionism), by contrast, establishes the science of psychology as a science in accordance with the canon of inductive philosophy as much as possible, i.e., as “the science of the elementary laws of mind” (Mill [1843] Reference Mill1882, 602).Footnote 19 Helmholtz definitely aligned with Mill to a certain extent in thus conceiving of the aim of psychological investigation. In contrast to nativism, he argued that his empirical theory “does not assume any peculiar modes of physiological action in the nervous system, nor any hypothetical anatomical structures” but instead considers the idea of the external in perception to be the result of an “act of our power of realization [Vorstellungsvermögen]” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 194, 127).
In itself, however, Helmholtz reluctantly admitted that “in the present state of knowledge it is impossible to refute the intuition theory” on a factual basis. Probably, it is exactly at this point that Helmholtz met the philosophical difficulties he mentioned to his friend du Bois-Reymond (see “Introduction,” Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:17). In what follows two such arguments used by Helmholtz will be explored, as well as the way in which they resonate with John Stuart Mill's defense of the psychological method, namely the latter's (i) lack of metaphysical commitment, and (ii) its parsimonious nature.
(a) Nativism's (metaphysical) audacity.
One of Helmholtz's main arguments against nativism was that to the extent that it accepts that “complete ideas of objects are produced by organic mechanisms,” it “assumes a pre-established harmony between sensations and their objects” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 357; [1868] Reference Helmholtz and Cahan1995, 202; also see Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:17–18, 252). Consequently, Helmholtz considered the nativist hypothesis to be a naturalized version of absolute idealism, since it is founded upon the presupposition of a fundamental representative similarity between subject and object, i.e., the thesis that the subjective consciousness of externality is somehow represented on the level of physiological organization prior to experience (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:18; Lenoir Reference Lenoir and Cahan1993). Both the idealist claim of an essential identity of subject and object, and the famous Leibnizian thesis of an original correspondence between subject and object (like “two clocks or watches which perfectly agree”), amount to an unjustified a priori assimilation of internal and external states (Leibniz [1695] Reference Leibniz and Strickland2004, 77). Contrary to nativism, Helmholtz's empirical theory accepted that “congruence rests upon a fact which is obtained from experience only” (Helmholtz [1870] Reference Helmholtz and Cahan1995, 229), and therefore avoids metaphysical commitment:
The reader will see how the subject of our present enquiry involves the consideration of that far-reaching opposition between the system of philosophy which assumes a pre-existing harmony of the laws of mental operations with those of the outer world, and the system which attempts to derive all correspondence between mind and matter from the results of experience. (Helmholtz [1868] Reference Helmholtz and Cahan1995, 177)
In the same vein, Mill argued that those who take certain mental phenomena to be simple and immediate, do so “not as psychologists, but as ontologists.” He wrote that with regard to questions pertaining to the mind, philosophers stand over and against each other if it comes to their ontological superstructure:
That the constitution of the mind is the key to the constitution of external nature – that the laws of the human intellect have a necessary correspondence with the objective laws of the universe . . . – is the grand doctrine which the one school affirms and the other denies . . . . But this question is beyond the compass of psychology. (Mill [1859] Reference Mill and Mill1867, 101)
As will be outlined in section 3.1., this perspective prompted Mill to oppose intuitionist theories of the origin of the belief in an external world, as represented prototypically in Sir William Hamilton's 1859 Doctrine of the Natural Duality of Consciousness. For both Mill and Helmholtz, the question as to the origin of the perceptual object, and specifically the question of whether or not it is viable for psychological analysis, thus had a metaphysical significance.
(b) Nativism as a redundant hypothesis.
Another objection Helmholtz repeatedly raised against nativism is that it violates the “general . . . rule of scientific examination, not to build new hypotheses as long as known facts seem adequate for the explanation” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:17). The empirical theory, he argued, is not only less “audacious and dubious” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 357) than nativism, but also more economical, as it appeals only to the “the known laws of thought” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 194). To be sure, Helmholtz acknowledged that these laws might be somewhat mysterious, and that there is scarcely any hope that their existence will ever be established on a factual basis, as they remain deeply hidden in the human mind (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, and [1868] Reference Helmholtz and Cahan1995). Still, he maintained that “these [mental] operations actually exist, . . . these mysteries of the laws of thought cannot be regarded from a scientific point of view as constituting any deficiency in the empirical theory” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 194). A similar argument was presented by Mill in defense of the psychological theory of externality against the intuitionist alternative, when he stated for example that “the first of the laws laid down . . . for the interpretation of consciousness, [is] the law . . . of parsimony” (Mill Reference Mill and Mill1865, 242–243).Footnote 20
It could be doubted however, whether this is a pressing argument at all, as methodological empiricism replaces one hypothesis (the self-evident nature of the idea of externality), by another (the genetic nature of the idea of externality). The nativist could indeed wonder why he would consider quality and intensity of sensation to be primitive, and not external reference, as Liebmann (Reference Liebmann1869) argued. In the end, the empiricist is bound to fall back upon his principled decision for a particular mode of psychological explanation, and his anti-metaphysical argument, that prescribes that nothing can be accepted as a primitive fact, if it can be accounted for in genetic terms in principle. In itself, the genetic approach invokes as much auxiliary hypotheses as its alternative.
3. Helmholtz's Psychology of the Object
From what has been said thus far, it is clear that Helmholtz's motive for establishing the problem of the object as a psychological problem, derived partially from his expansion and neo-Kantian interpretation of LosNE on the one hand, and his methodological empiricism on the other. In turning to Helmholtz's psychology itself, however, the peculiar entanglement of philosophical perspectives becomes even more apparent. In what follows, the focus will shift to Helmholtz's psychological account of objectification, i.e., his investigation of “the conditions underlying the genesis of perceptions,” with the latter being defined as “conceptions [Vorstellungen] as to the existence, form and position of external objects” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:1).
On the one hand, this investigation involves a further exploration of the way in which Helmholtz's theorizing is historically embedded in methodological empiricism, or non-materialistic naturalism with regard to the mind. While it has been suggested that Helmholtz's theory of perception as unconscious inference is remarkably similar to Mill's Psychological Theory of the Belief in an External World, this hypothesis has remained largely unexplored (Mill Reference Mill and Mill1865, 234–250; Boring Reference Boring1950; Hochberg Reference Hochberg and Peterson2007). As this could provide valuable insight into Helmholtz's position as a proto-psychologist, however, section 3.1 will present a comparative analysis of Helmholtz's psychology of perception and Mill's empirical psychology of objectification. For one thing, this should put us in a better position to understand why Hochberg for example, talks explicitly of the “Helmholtz-Mill theory of object perception” (quoted in Peterson, Gillam, and Sedgwick Reference Peterson, Gillam and Sedgwick2007, 331).
In contrast to Mill, however, Helmholtz denied that the genesis of the perceptual object can be accounted for exhaustively by means of “nothing but a mechanically acquired association of ideas,” and instead founded his empirical theory on elements that resonate with Kant's and Fichte's transcendental analyses of experience (Helmholtz [1855] Reference Helmholtz1896, 114–115), as will be outlined in section 3.2. In contrast to the ongoing debates on Helmholtz's allegiance to Kant's critical philosophy, the continuity of important aspects of his thought with Fichte's philosophy has received rather minimal scholarly attention.Footnote 21 Amongst the few who have pointed out a certain allegiance of Helmholtz's psychological theory of perception to Fichte's ego-doctrine are Turner (Reference Turner1977), Scheerer (Reference Scheerer1989), Schulz (Reference Schultz2004), Meulders (Reference Meulders2010) and especially Heidelberger (Reference Heidelberger and Cahan1993 and Reference Heidelberger and Krüger1994). With the exception of the latter, however, these interpretations remain fragmentary and vague, with comments on the matter being restricted to no more than a few tentative statements. Heidelberger is to be credited with putting the matter of Helmholtz's allegiance to Fichte's Ego-doctrine on the agenda as an area of interest in its own right, by convincingly arguing that the appreciation of Fichte's work forms “the essential key to understanding Helmholtz's philosophy of science” (Heidelberger Reference Heidelberger and Cahan1993, 495).Footnote 22
3.1 Perception as Unconscious Inference
In attempting to get a firmer grasp of the empirical dimension of Helmholtz's psychology of the object, Mill's Reference Mill and Mill1865Psychological Theory of the Belief in an External World provides an excellent starting point, not only because it provides a prototypical example of a strict empiricist account of the perceptual object, but moreover (and more importantly) because of the parallels with Helmholtz's theory of perception as unconscious inference.
First and foremost, Mill's psychological theory was meant to provide an alternative to common-sense philosopher William Hamilton's Doctrine of the Natural Duality of Consciousness, which stated that “we may . . . lay it down an undisputed truth, that consciousness gives, as an ultimate fact, a primitive duality; – a knowledge of the ego in relation and contrast to the non-ego . . . The ego and non-ego are, thus given in an original synthesis” (Hamilton Reference Hamilton and Mansel1859, 202–203). On the basis of the arguments presented in section 2.2., Mill, in his turn, set out to show that (i) it is possible to analyze the belief in an external world in terms of its formative history in the mind, and (ii) that this empirical analysis could furthermore account for the apparent intuitive nature with which the idea of the object emerges in the mind. In Mill's own formulation, his psychological theory of the object aims at demonstrating that:
there are associations naturally and even necessarily generated by the order of our sensations and of our reminiscences of sensation, which, supposing no intuition of an external world to have existed in consciousness, would inevitably generate the belief, and would cause it to be regarded as an intuition. (Mill Reference Mill and Mill1865, 236)
Only two general presuppositions are needed, according to Mill, for his psychological account, namely (1) that “the human mind is capable of expectation,” i.e., that it can conceive of future possibility, and (2) the laws of association (Mill Reference Mill and Mill1865, 234).Footnote 23 Based on these two postulates, Mill hypothesized that the notion of the object as an independent existence in perception, arises from the recognition of a difference between present and possible sensations. He gives the following example:
I see a piece of white paper on a table. I go into another room. . . . though I have ceased to see it, I am persuaded that the paper is still there. I no longer have the sensations which it gave me; but I believe that when I again place myself in the circumstances in which I had those sensations, that is, when I go again into the room, I shall again have them; and further, that there has been no intervening moment at which this would not have been the case. . . . The conception I form of the world existing at any moment, comprises, along with the sensation I am feeling, a countless variety of possibilities of sensation. (Ibid., 237)
In other words, if we say that consciousness contains the belief in objective existence, we are saying no more than that apart from the flow of actual sensations, we recognize a permanent possibility of sensation as a conditional certainty. Or in short: the object itself is no more than the permanent possibility of sensation (ibid., 249). Later, Mill adds, we come to identify these permanent possibilities with the idea of cause, and hence the conception of the possibility of sensation as the notion of the object, is hypostasized as the external cause of subjective modifications. In conclusion, Mill argued:
the very idea of anything out of ourselves is derived solely from the knowledge experience gives us of the Permanent Possibilities. Our sensations we carry with us wherever we go, and they never exist where we are not; but when we change our place we do not carry away with us the Permanent Possibilities of Sensation: they remain until we return, or arise and cease under conditions with which our presence in general has nothing to do. . . . and when the idea of cause has been acquired, . . . nothing can be more natural than that the Permanent Possibilities should be classed by us as existences generically distinct from our sensations, but of which our sensations are the effect. (Ibid.)
To account for the direct, intuitive nature with which this belief emerges in consciousness, Mill introduced “the principle of obliviscence,” which states that not all the elements of an associational chain leading up to the generation of an idea need to appear in consciousness.Footnote 24 By virtue of this principle, “we see, and cannot help seeing, what we have learned to infer,” i.e., the idea of the permanent possibility of sensation (the object), that has been acquired through associational learning, will appear to be intuitive, although it is actually acquired (ibid., 236).
If we now compare Helmholtz's theory with Mill’s, the parallels are striking at first. Take for example the following statement from 1878:
Let us call the entire group of aggregate sensations induced . . . by a certain definite and finite group of the will's impulses the “current presentables”; by contrast, let us call “present” the aggregate of sensations from this group which is just coming to perception. Our observer . . . can make each individual presentable present to himself at any moment through execution of the relevant movement. In this way it seems to him that each individual from this group of presentables exists at each moment during this period of time. . . . Thus the idea of a simultaneous and continuous existence of different things alongside one another will be achieved. (Helmholtz [1878] Reference Helmholtz and Cahan1995, 350)
From this passage, it seems that for Helmholtz too, the idea of external existence was entirely derived a posteriori from the recognition of something like Mill's “permanent possibilities,” based on the learned association between the coming into being of certain “circles of presentables” and voluntary movement. Once this kind of knowledge is acquired, Helmholtz added, the perceptual process takes the form of an unconscious inductive syllogism, with the current sensation as a minor premise, acquired knowledge of lawlike covariation between movement and sensation as a major premise, and the object as a conclusion (Helmholtz [1868] Reference Helmholtz and Cahan1995, 199). Based on the tacit knowledge expressed in the major premise, the perceptual process is thus entirely regulated by the rule that “such objects are always imagined as being present . . . as would have to be there in order to produce the same impression on the nervous mechanism” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:2; emphasis added). As such, the perceptual process takes the form of a continuous process of hypothesis testing, based on expectations generated by previous experience. Consequently, Helmholtz called his theorizing a projection theory: throughout the perceptual process, expectations are constantly being projected onto the visual field, and give rise to perceptual images of the form, position, and existence of objects.
With respect to this particular conceptualization of perceptual experience, Helmholtz actually acknowledged his indebtedness to Mill's Logic, which, according to him “gave the best explanation” of the nature of inductive conclusions [Inductiven Schlüsse], in conceiving major premises not as universal truths, but as a collection of acquired knowledge derived from similar cases (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:24). Furthermore, Helmholtz's notion of the unconscious fulfilled the same systematic role as Mill's principle of obliviscence, i.e., accounting for the seemingly intuitive and direct nature of perception. In the Treatise, Helmholtz summarized his account as follows:
we can never emerge from the world of our sensations to the apperception of an external world, except by inferring from the changing sensation that external objects are the causes of this change. Once the idea of external objects has been formed, we may not be concerned any more as to how we got this idea, especially because the inference appears to be so self-evident that we are not conscious of its being a new result. (Ibid., 2:32)
In conclusion, there is no doubt that there is a certain degree of similarity between Mill's and Helmholtz's psychological approaches, and that Helmholtz certainly appears to be a “disciple of the English empirical philosophers,” and more particularly of Mill, if it comes to his analysis of the object (Meulders Reference Meulders2010, 143). This is an observation that prompted Hochberg to talk of the Helmholtz-Mill theory of perception (Peterson, Gillam, and Sedgwick Reference Peterson, Gillam and Sedgwick2007, 331). However, a complete reduction of Helmholtz's theorizing to Mill's psychological theory does not do justice to the complexity of his psychological project in general, and to the way in which his empiricism is embedded in reflections on the a priori constitutive (as opposed to constructive) elements of experience, inspired by the transcendental tradition.
3.2 Helmholtz and the Transcendental Tradition
In sharp contrast to Mill – who never ceased to contrast his program for an empirical approach to mental phenomena with a priori philosophy – Helmholtz was never hostile towards the German transcendental tradition as such, but on the contrary, related his own work on sense perception explicitly to the theories of Kant and Fichte.Footnote 25 Some of the most valuable insights in Helmholtz's intellectual relation to the German transcendental tradition, are to be found in the correspondence between him and his father Ferdinand, who had studied philosophy and philology, was personally acquainted with Schopenhauer, and close friends with Immanuel Hermann Fichte, the son of Johann Gottlieb. It is interesting to note that the latter was furthermore Helmholtz's godfather and namesake.Footnote 26 In his Autobiographical Sketch, Helmholtz testified:
The interest for questions of the theory of cognition, had been implanted in me in my youth, when I had often heard my father, who had retained a strong impression from Fichte's idealism, dispute with his colleagues who believed in Kant or Hegel. (Helmholtz [1891] Reference Helmholtz and Cahan1995, 390)
Although Helmholtz endorsed an explicitly anti-metaphysical position, he likewise emphasized that “philosophy, if it gives up metaphysics, still possesses a wide and important field, the knowledge of mental and spiritual processes and their laws” (Helmholtz [1877] Reference Helmholtz and Cahan1995, 325). As such, he considered his psychological project to be in alignment with “Kantian philosophy, that aimed at . . . investigating the sources of knowledge and the degree of their justification” (Helmholtz [1855] Reference Helmholtz1896, 88).”Footnote 27 In the early 1850s, when Helmholtz was appointed a professorship in anatomy and physiology in Bonn, and had just started his extensive research on the nature of human perception, Helmholtz wrote to his father:
It seems to me a favorable moment for voices of the old school of Kant and . . . Fichte to obtain a hearing once more. . . . Philosophy finds its great significance among the sciences as the theory of the source and functions of knowledge, in the sense in which Kant, and, so far as I have understood him, the elder Fichte, took it. (Letter from December 31th, 1855, as quoted in Koenigsberger Reference Koenigsberger1902/03, 284)
Later on in his career, Helmholtz likewise took on every occasion to stress the way in which his research confirmed or continued aspects of Kant's and Fichte's philosophy (see for example Helmholtz [1868, 1878, 1891, 1892] 1995). However, it is sufficiently clear that he read both authors more as pure psychologists, than as epistemologists, and that hence, he transformed their formal investigations into hypotheses with regard to the necessary psychological hardware underlying the capacity to perceive (see below).
Nevertheless, from a systematic viewpoint, Helmholtz's “decisive intellectual leap” to the transcendental tradition in the context of his psychological investigation is hardly surprising (Helmholtz [1892] Reference Helmholtz and Cahan1995, 410). A particular hallmark of Mill's psychology of the object, for example, is that the question as to the origin of representation is dissociated from that of the representing subject. Hence, his theory was soon criticized for not taking into account the active subject that was actually presupposed in his theory of the object as a permanent possibility of sensation (see for example Bradley Reference Bradley1876; James Reference James1890). Helmholtz, on the other hand, credited Kant's first Critique as reflecting an epochal shift in thinking about human experience, as it demonstrated the constitutive role of “pure thinking a priori,” and the way in which all of our perceptions are “conditioned by the peculiar ability of our mind [eigenthümlichen Fähigkeiten unseres Geistes]” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:35). Furthermore, he added: “according to this view perception is recognized as an effect [Wirkung] produced on our sensitive faculty . . . this effect . . . being just as dependent on what causes the effect [dem Wirkenden] as on the nature of that on which the effect is produced [dessen, auf welches gewirkt wird]” (ibid., 3:35–36). Hence, Helmholtz described his theory of perception as constituting a borderland [Grenzgebiet] between the exact sciences and philosophy, in which the latter “considers . . . that which belongs to the mind's own activity. . . . that which is definition, designation, form of representation and hypothesis” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 344).
If we take Helmholtz's 1855 statement that “seeing is understanding sensation” as a point of departure in the shift from an empirical to a transcendental level of analysis, his theorizing can be understood as a shift from the interrogation of the structure of perceptual comprehension (unconscious inference), to an inquiry into the conditions of comprehensibility (Helmholtz [1855] Reference Helmholtz1896, 99–100; also see Helmholtz [1878] Reference Helmholtz and Cahan1995; and 1896). Or put more generally: as a shift from an inquiry of the mental realm as a “theatre’” for empirical construction, to an interrogation of what the mind or subject must be like a priori in order to be able to associate its way into the external world. In analyzing Helmholtz's psychology of the object from this perspective, two such “transcendental” questions – respectively associated with the Kantian and the Fichtean dimension in Helmholtz's psychology of the object – are especially prominent, that can be denoted as the problem of reference on the one hand, and that of difference on the other (discussed in sections 3.2.1 and 3.2.2 respectively):Footnote 28
(i) The problem of reference, or the question as to the ground of the signaling function of the sensation-sign: given the physiologically underdetermined nature of sign-sensations with regard to their origin, unconscious inference based on previous experience alone does not explain “what first enables us to pass from the world of nervous sensations into the world of actuality [Welt der Wirklichkeit]?” (Helmholtz [1855] Reference Helmholtz1896, 115–116; emphasis added). In other words: unconscious inference in itself does not account for the signaling function of the sign, or for the way in which a perceiver comes to interpret sensations as signs for something that is not contained in the sensation itself.
(ii) The problem of difference, or the ground of the dual structure of perceptual consciousness: the perceptual process presupposes a differentiation between those sensations referring to external objects or events, and those that are endogenous in origin. Therefore, Helmholtz investigated what “first makes the distinction between thought and reality possible [Scheidung von Gedachtem und Wirklichem erst möglich wird]?” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 362; emphasis added). One might say that this problem is even more fundamental than the previous one, as it pertains to the conditions of possibility underlying the ability to differentiate between those sensations that signify objects, while others originate from the mind or body of the perceiver.
3.2.1 Helmholtz, Kant, and the Problem of Reference
In the context of discussions regarding Helmholtz's intellectual indebtedness to German philosophy, a much debated topic is the former's appropriation of Kant's a priori view with respect to the causal law. As early as 1855, Helmholtz rejected the empiricist regularity view of causality, and claimed instead that the causal structure of understanding is an a priori condition for experience:
we have to presuppose the presence of external objects as the cause of nerve excitation, since there can be no effect [Wirkung] without a cause. How do we know that there can be no effect without a cause? Is it a law of experience [Erfahrungssatz]? According to some, it is. But . . . we use the law to arrive . . . at the insight that there are objects in space in the first place . . . . Can we get it from the internal experience of our self-consciousness? No; since we conceive of self-conscious acts of volition and thought as free; i.e., we deny that they are the necessary effects of sufficient causes. In this way, the inquiry into the nature of sense perception leads us to the insight that Kant had already revealed: that the law . . . is a law of thought, given prior to experience. (Helmholtz [1855] Reference Helmholtz1896, 116; see also Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:33; [1878] Reference Helmholtz and Cahan1995, 363; 1896, 590)
Hence, Helmholtz held causality to be an a priori form of thought [Denkform], rather than a content derived from experience, and the scientist explicitly rejected Mill's regularity view of causation, that claims that “the law of cause and effect, is itself an instance of induction” (Mill [1843] Reference Mill1882, 401).Footnote 29 Later on in his career, Helmholtz maintained that “the causal law is really an a priori given, a transcendental law. It is not possible to prove it by experience” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 363).
In Helmholtz's psychological theory, the transition of “the world of nervous sensations” to that “of external objects,” is crucially mediated by the a priori rule that every effect has a cause, and hence, Helmholtz's Kantianism in this respect, provided a way out of the problem of reference. That is to say: without the assumption that “there is no effect without a cause [keine Wirkung ohne Ursache],” sensation would refer to nothing but itself, and hence, perception would be utterly impossible (Helmholtz [1855] Reference Helmholtz1896, 116).
Although Helmholtz was quite explicit about the Kantian pedigree of his conception of the causal law, the purport and soundness of his self-professed Kantianism is (and has been) a notorious apple of discord in secondary literature. For starters, Helmholtz considered the law to be a priori in a fairly outspoken psychological sense, describing it, amongst others as an “urge [Trieb] of our intellect [Verstandes]” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:34), later defining “Trieb” as “the inborn effect of our organization” (Helmholtz Reference Helmholtz1894, 96). The causal law in Helmholtz's theory of perception expresses a psychological disposition, and more particularly, the subjective urge to objectify underdetermined sensory modifications by assigning them an origin. Through this interpretation, however, the Kantian a priori apparently ceased to be a merely formal (logical) condition of experience, but instead expressed a necessary presupposition regarding a perceiver's psychological hardware. In this sense, it seems reasonable to presume that “Helmholtz reinterpreted the Kantian a priori as something that Kant never intended it to be: a species of psychological adaptation” (Disalle Reference DiSalle and Cahan1993, 505).
Furthermore, although Helmholtz never abandoned his view on causality as psychological necessity, he did emphasize its epistemological contingency.Footnote 30 In a posthumously published note, for example, Helmholtz claimed that the law is “a mere hypothesis, and not otherwise demonstrable” (Helmholtz as quoted in Koenigsberger Reference Koenigsberger1902/03, 247). In addition to his psychological interpretation of the law, Helmholtz thus diverged from Kant at the point where he dissociated the questions of necessity and objectivity, which were inextricably linked in the latter's critical project (Fullinwider Reference Fullinwider1990, 48). From this, Schiemann concludes that Helmholtz's conception of causality in his later years “no longer deviated much from Mill's standpoint on the issue” (Schiemann Reference Schiemann2009, 240). In the same vein, Riehl and Schlick argued earlier in a footnote added to his translation of Helmholtz's The Facts in Perception, i.e., that the latter “took the path of David Hume” with regard to causality, since its validity never rises above its status as a mere hypothesis (Riehl Reference Riehl1904; Helmholtz Reference Helmholtz, Herz and Schlick1921, 163). If we restrict ourselves to Helmholtz's mature insistence on the hypothetical character of causal inference – and thus on the epistemological status of causality in Helmholtz's work – this might indeed be a viable hypothesis. It remains, however, that Helmholtz maintained that causal understanding is a necessary a priori ingredient of experience that is not derived from induction, but rather forms its very condition of possibility. Furthermore, the scientist reiterated his Kantian stance on the matter in 1878 as well as in the second, revised version of the Treatise (Helmholtz [1878] Reference Helmholtz and Cahan1995, 363; 1896, 590). These observations complicate and challenge readings of Helmholtz's statements in this respect as testifying of a strict empiricist viewpoint, although it cannot be denied that Helmholtz's reduction of the a priori to a psychological necessity diverges significantly from Kant's conception.
It is important to note that the aprioricity of the causal law can be invoked to solve the problem of reference, but it does not account for the way in which internally and externally generated sensible effects are differentiated in perception, or for the question as to the origin of differential consciousness. This is a problem for which, according to Helmholtz, even Kant's system is of “virtually no help” (Helmholtz [1892] Reference Helmholtz and Cahan1995, 394).
3.2.2 Helmholtz, Fichte, and the Problem of Difference
As already suggested, the limits and scope of Helmholtz's indebtedness to the transcendental tradition cannot be assessed properly, without taking into account the continuity of his theorizing with the philosophical system of the post-Kantian J.G. Fichte. A compelling argument against any interpretation of Helmholtz's psychological theory along the lines set out by Fichte, would be that the scientist's anti-metaphysical attitude was irreconcilable with the quite robust metaphysical and speculative character of post-Kantian philosophy in general, and Fichte's system in particular. This objection cannot be dismissed, but as Turner pointed out, Helmholtz apparently distinguished between “two aspects of Fichte's thought,” and accepted Fichte's philosophy to the extent that it ”represented a phenomenology of consciousness” although it can hardly be denied that he “resolutely rejected Fichte's . . . attempts to build an idealist metaphysics on that basis” (Turner Reference Turner1977, 57). This aligns with Helhmholtz's claim that in contrast to Hegel, Fichte's philosophy “does not . . . contradict the natural sciences, but rather . . . corresponds precisely to . . . the conclusions . . . of physiology” (Helmholtz [1855] Reference Helmholtz1896, 89).
Helmholtz's lectures, as well as the correspondence with his father, provide some further clues on this matter. When writing to his father on the topic of his 1855 lecture On Human Vision, he confided:
Last Tuesday . . . I gave another lecture upon Human Vision, in which I tried to put forward the correspondence between the empirical facts of the physiology of the sense-organs and the philosophical attitude of Kant, and also of Fichte, although I was somewhat hindered in my philosophical exposition by the need of making it popular. (Helmholtz in a letter to his father from February 27th 1855, as quoted in Koenigsberger Reference Koenigsberger1902/1903, 242)
In yet another letter, Helmholtz even went as far as claiming that his theory can be read as “an empirical statement of Fichte's fundamental views of sense-perception” (ibid., letter from September 1852, 92). Furthermore, Helmholtz referred approvingly to Fichte's 1817 paper on The Facts of Consciousness [Die Thatsachen des Bewusstseins] in the second, revised version of his Treatise, and claimed that it contains “a lot of right, strongly worded” insights on sense perception (Helmholtz Reference Helmholtz1896, 2:248).
To understand the possible relevance of Fichte's philosophy for Helmholtz's psychology of the object, we should first turn to the latter's research on eye movements, performed mainly between 1862 and 1865, when Helmholtz was a professor in physiology in Heidelberg (Koenigsberger Reference Koenigsberger1902/03). As was explained in section 3.1., Helmholtz conceived of the perceptual process as crucially mediated by acquired knowledge on the lawlike covariation between voluntary movement and the coming into being of certain “circles of presentables,” or in short, by anticipative procedures. Hence, this knowledge [Kennen], entails what Helmholtz in turn called Können [being able to], i.e., it requires that a perceiver is “acquainted with the particular innervation of muscles, which is necessary in order to produce any effect we intend” (Helmholtz [1868] Reference Helmholtz and Cahan1995, 198). In his Treatise, Helmholtz further developed this physiological understanding of the nature of voluntary movement, by hypothesizing that every voluntary act is represented on the level of sensibility as a complex of three sensations, that, when taken together, constitute what he calls the “muscular feeling” [Muskelgefühl]:
This term includes . . . several different sensations that have to be distinguished . . .:
1. The intensity of the effort of will [Intensität unserer Willensanstrengung], whereby we endeavor to bring the muscles in action,
2. The tension of the muscles . . .,
3. The result of the effort, which . . . makes itself felt in the muscle by a contraction which actually takes place. (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:243)
While the last two sensations denote afferent (or centripetal) physiological markers for agency, the first sensation is a centrifugal, efferent signal that precedes the action. As such, Helmholtz's theorizing is crucially founded in the assumption that actions are known by the subject “otherwise than by their effect,” as Jeannerod once put it, i.e., that the feeling of activity or spontaneity is generated centrally, preceding the actual muscle movement (Jeannerod et al. Reference Jeannerod1979, 241). Although Helmholtz called the awareness of Willensanstrengung a sensation, strictly speaking, it is not sensed in the same way as other sensible events, i.e., it is not produced, but rather presupposed by the sensations associated with muscle contraction and its results. Based on this physiological operationalization of the concept of voluntary action, the scientist hypothesized that a particular malfunctioning in either one of the sensations related to voluntary movement, could cause subnormal perceptual experiences (illusion and hallucination) (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 2:§29). One of the most significant observations Helmholtz described in this respect, are the perceptual phenomena experienced by subjects suffering from a partial or total paralysis of the eye muscles, in which the complex of muscular feeling is reduced to the sensation of the effort of will only:
if the external rectus of the right eye is paralyzed, or the nerve leading to it, this eye can no longer be pulled around to the right. . . . the moment he tries to turn his eye outwards, that is, to the right, it ceases to do his bidding, and remains standing in the middle, while the objects appear to move to the right. . . . we fancy that the visual axis has been shifted to the right; and since no change has taken place in the positions of the images on the retina of the paralyzed eye, we get the impression that the objects shared the supposed movements. (Ibid., 245)
In these cases, the perceptual process is guided only by a psychological intention, that, however, remains unrealized due to paralysis, and thus leads to anomalous projections onto the visual field. But Helmholtz likewise described the opposite case: if the eye is moved to the right by some external force – and hence none of the sensations mentioned above would be produced – an illusory perception of motion is experienced.Footnote 31 In this latter case, the shifting retinal image cannot be ascribed to self-induced movement, and hence, is externalized, although no actual movement is taking place (ibid., 31). Through these observations, Helmholtz grew increasingly convinced of the indispensable role of intentions – physiologically described in terms of the efferent sensation of Willensanstrengung – in object perception on the one hand, and the physiological irreducibility of voluntary movement to muscular contraction on the other.Footnote 32
The crucial role of the awareness of the effort of will in Helmholtz's theorizing, is, however, not restricted to the important share it has in judgments about the spatial position of external objects, but on the contrary, is constitutive of the ability of objectification itself, and more precisely: of perceptual differentiation. The process of unconscious inference, through which sensations are relegated to their supposed external causes, is based upon the more fundamental experience of a dynamical opposition between “those changes which are cause by act of our will, from those that cannot be ascribed to our will” (Helmholtz Reference Helmholtz1896, 592–593). Without the immediate sensory awareness of “the intensity of the feeling of effort [Intensität unserer Willensanstrenung],” Helmholtz wrote, no distinction is possible between internally and externally generated sensory modifications:
it is clear that the separation of thought and reality first becomes possible after we know how to complete the separation of that which the ego can and cannot change. This however, only becomes possible after we recognize which law-like consequences the will's impulses have at that time. (Helmholtz [1878] Reference Helmholtz and Cahan1995, 362)
This is the way in which Helmholtz's theory of agency provided an answer to what was previously described as the problem of difference: the object in perception first appears first as a negation of that which can be ascribed to the self; the “self” in turn being a function of the autonomous, and immediately perceived will's impulse. As such, differentiated consciousness is first generated according to the scheme of will and resistance. Crucially, Helmholtz added that this account is founded on two foundational assumptions. First of all, “the impulse to movement . . . is something directly perceivable. We feel that we do something . . . We do not know directly what we do” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 348). Secondly, Helmholtz maintains that it is an “essential assumption . . . that our will's impulse has neither already been influenced by physical causes, . . . and not itself psychically influenced the succeeding perceptions (ibid., 358–359). Hence, Helmholtz's theorizing was founded on the directly perceivable, and autonomous nature of the Willensanstrengung, which he furthermore described as a psychical (not a physiological or physical) act of will (ibid., 359). To summarize, Helmholtz maintained that it is only through the voluntary act that we learn to distinguish:
those changes which we can bring forth and annul by conscious impulses of the will . . . from those which are not consequences of the will's impulses and cannot be overcome by such. The latter finding is negative. Fichte's appropriate expression for it is that a Non-Ego forces recognition of itself vis-à-vis the Ego. (Ibid., 351)
In the end, this voluntarist tenet in Helmholtz's theory prevents a complete reduction of the perceptual process to mechanical law-like cognitive mechanisms. In fact, Helmholtz claimed that “in ascribing to ourselves free-will . . . we deny in toto the possibility of referring at least one of the ways in which our mental activity expresses itself to a rigorous law” (Helmholtz [1862] Reference Helmholtz and Cahan1995, 85). Furthermore, it suggests that a theory of experience should necessarily take an active, goal-oriented subject as a point of departure. Indeed, the scientist stated, “the efforts of the philosophical schools to establish belief in the existence of reality must remain unsuccessful so long as they proceed only from passive observations” (Helmholtz [1892] Reference Helmholtz and Cahan1995, 410).
As the above quoted passage from Helmholtz – as well as the fragments from the letters written to his father as presented in the beginning of this section – makes clear, the scientist related this dimension of his theory explicitly to Fichte's philosophical system. Most generally, the latter had grounded his transcendental analysis of experience in the concept of the I as a pure self-relating activity [Tathandlung], and claimed that it is only for an I thus conceived, that an object can be posited as a Gegenstand or resistance: “no object can be posited without the presence of an activity [Tätigkeit] of the I, that is opposed to the activity of the object” (Fichte [1794] Reference Fichte1997, 177). In this sense, the object or Not-I in Fichte's system, first becomes intelligible as a negation of the I's unconditioned centrifugality, i.e., as resistance: “The object [Gegenstand] is posited, in so far as an activity of the I is resisted; if there is no such activity of the I, there is no object” (ibid., 259). Consciousness is thus said to be constituted as a dual structure, through the reciprocal determination of I and Not-I, dialectically related to each other as reality and negation, posited through the awareness of limitation or resistance. For Fichte, the subject's centrifugal activity was the alpha and omega of experience, as is testified by his definition of the intellectual intuition that underlies the possibility of external consciousness:
the immediate consciousness that I act . . . . I can take no step, cannot move a hand or a foot, without the intellectual intuition of my self-consciousness in these acts; it is only by virtue of this intuition, that I know, that I am doing this, it is only by virtue of this [intellectual intuition] that I distinguish my act . . . from the . . . object of my act. (Fichte Reference Fichte1798, 43; emphasis in the original)
Notwithstanding compelling evidence suggesting that Fichtean philosophy was actually central to Helmholtz's thought, Heidelberger (Reference Heidelberger and Cahan1993, Reference Heidelberger and Krüger1994) is the only one to have analyzed this interesting continuity, although the latter focused specifically on the Fichtean dimension in Helmholtz's philosophy of science.Footnote 33 This perspective differs from the psychological angle taken in this exposition, although it could be said that Helmholtz's view on the constitutive role of experimentation unites his epistemological and psychological stance, as is clear for example from the following statement:
The same great importance which experiment has for the certainty of our scientific convictions it has also for the unconscious induction of the perceptions of our senses. It is only by voluntarily bringing our organs of sense in various relations to the objects, that we learn to be sure as to our judgments of the causes of our sensations. (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 3:30–31; see also McDonald Reference McDonald2003)
It is interesting to note that Helmholtz's definition of truth or objectivity, was likewise derived from the pivotal role he assigned to voluntary action: “We call those ideas of the external world true,” the scientist wrote, “which inform us in a sufficient manner about the consequences of our actions in the external world, and allow us to judge correctly on what changes to expect” (Helmholtz Reference Helmholtz1896, 590). Or at another point in time, he stated that “there can be no possible sense in speaking of any other truth of our ideas” than that which allows us to “regulate our movements and actions” (Helmholtz [Reference Helmholtz and Southall1856/66] 1924/25, 2:19). Hence, the experiment has become a constitutive element not only for the perceptual process, but likewise for the notion of objectivity itself. This challenges Daston and Galison's claim that objectivity in the modern era arose from a negation of subjectivity, and more specifically, as an attempt to counterbalance the “willful self . . . perceived as endangering scientific knowledge” (Daston and Galison Reference Daston and Galison2007, 37).Footnote 34 Quite the contrary, what this analysis has pointed out, amongst other things, is that Helmholtz's views on the object and objectivity did not so much attempt to neutralize the willful subject, as it established the latter as a condition of possibility for object constitution.
To conclude this section, it should be noted that the foregoing analysis might shed some new light on the much debated issue of Helmholtz's rejection of Kant's transcendental theory of space. It is well known that Helmholtz, in contrast to Kant, argued that the axioms of geometry are empirical.Footnote 35 More particularly, Helmholtz contended that Kant deemed spatial determination to be irreducible to a mental process, while it can in fact be fully accounted for in terms of (a posteriori) psychological construction, mediated by voluntary movement:
It is easy to appreciate that by moving the . . . finger along the objects, the sequence in which the impressions of the object are presented becomes known; that this sequence shows itself to be independent of whether one feels with this or with that finger; . . . and different tangible surfaces require different motions in order to glide along them . . . . In such a way may knowledge of the spatial ordering of things existing beside one another be acquired. . . . This observed spatial order of things originally derives from the sequence in which the qualities of the sensation present themselves to the moved sensory organ. (Helmholtz [1878] Reference Helmholtz and Cahan1995, 351–352)
Helmholtz thus accounted for the metric determination of space in terms of an “unconscious mathematics” (Cassirer Reference Cassirer1944, 18). It is important, however, to understand that Helmholtz argued specifically against the apriorism with regard to the (Euclidian) structure of space, while he maintained that the Kantian hypothesis of space as the general form of external intuition, emptied from its structural specifications, is “completely inoffensive”:
Kant's theory of the a priori given forms of intuition is a very apt and clear expression of the relations of things; but these forms must be without content and sufficiently free . . . . The axioms of geometry . . . limit the form of intuition of space in a way such that if geometry is to be generally applicable to the real world, then no longer can any imaginable content be included in it. If we eliminate the axioms, then the theory of the transcendality [sic] of the form of intuition of space is completely inoffensive. Kant has here, in his Critique, not been critical enough. (Helmholtz [1878] Reference Helmholtz and Cahan1995, 380)
Hence, Helmholtz specified that “space can be transcendental without there being any axioms” (ibid., 369). In other words, according to Helmholtz, Kant erred not so much in assuming that space in general is the necessary a priori form of intuition – i.e., in assuming the apriocity of a “purely formal scheme” – but rather in including “certain peculiarities of the scheme” in his transcendental theory (Helmholtz [1870] Reference Helmholtz and Cahan1995, 226). The latter claim can be better understood when placed against the background of Helmholtz's psychology of the object. Helmholtz articulated the basic structure of perceptual consciousness primarily in terms of a geometrical opposition, stating for example that “we understand as the external world precisely what we perceive as spatially determined. That which has no perceptible spatial relation, we conceive as the world of inner intuition, as the world of self-consciousness” (Helmholtz [1878] Reference Helmholtz and Cahan1995, 349). As such, the problem of space, for him was inherently related to what we called the problem of difference, and in that sense, Neuber is quite right in arguing that Helmholtz's theory of space was essentially a theory of space perception (Neuber Reference Neuber2012, 168–169). Upon closer analysis the transcendental element in Helmholtz's theory of space is not so much eliminated, as it is shifted to the constitutive role of the possibility of free mobility of rigid bodies (Helmholtz [1870] Reference Helmholtz and Cahan1995; Neuber Reference Neuber2012). Footnote 36 Specifically within the context of his perceptual theory, Helmholtz stated that:
space would be a . . . form of intuition priori to all experience insofar as its perception would be tied to the possibility of the will's motoric impulses, and for which the mental and corporeal ability must be given us through our organization before we can have spatial intuition. (Helmholtz [1878] Reference Helmholtz and Cahan1995, 349)
As such, one could hypothesize that Helmholtz, after relocating the possibility of spatial construction from the (a priori) form of external intuition to its (a posteriori) content, identified the aprioricity of space in general (as the necessary form of external perception) with the possibility for voluntary movement. In this sense, Helmholtz's theory was as much a criticism and refutation of Kant's a priorism with respect to spatial structure, as it is a rethinking of the meaning of the general concept of space as a transcendental form. To be more precise, in the end, the necessary condition of external intuition in Helmholtz was not space, but the capacity for voluntary movement that is claimed to found the possibility of spatial construction and objectification.
4. Discussion and Conclusion
The investigation of the scientific and philosophical background of Helmholtz's psychology of perception has provided further evidence for the profound entanglement of empiricism and idealism in Helmholtz's work, which has aroused scholarly interest ever since the nineteenth century. On the one hand, this paper aimed at broadening the scope of these discussions by pointing out the important continuity between Helmholtz's theorizing and the philosophy of J.S. Mill and J.G. Fichte, figures who had hitherto been treated unjustly as having only minor import for the interpretation of Helmholtz's work. More importantly, Helmholtz's “dovetailing” between, or attempting to reconcile empiricism and idealism has been examined specifically from the perspective of his psychological project, which has allowed us to grasp the systematic significance of Helmholtz's empirico-transcendentalism specifically from within the context of his attempts to address questions pertaining to the mind.
In focusing on the systematic place of Helmholtz's empirico-transcendentalism within his psychological project, one could hypothesize that his inclination towards transcendental philosophy correlated with a concern for the epistemic subject of perception, and a correlative interrogation of what the subject must be like in order to be able to construct an external world – or to address the questions of reference and difference – from the chaos of underdetermined sensations. Most generally, it is exactly at the point where Helmholtz supplemented his psychological investigation into the genesis of objective representation, with a transcendentally inspired interrogation of the representing subject, that his psychology overstepped the limits of the strict empiricist framework. Helmholtz's empirical theory of perception as unconscious inference hinged on assumptions that are not in any way derived from experience – i.e., the causal structure of understanding and the autonomous, undetermined will's impulse – but on the contrary, are to be presupposed to account for the empirical construction of the world.
An appreciation of the voluntarist dimension in Helmholtz's account could help restore the continuity of his psychophysiological project with early psychological theories, and most notably with Wilhelm Wundt's voluntarist psychology (see especially Wundt Reference Wundt1862). Although both Helmholtz and Wundt studied with Johannes Müller, and furthermore worked together for some years in Heidelberg, it has become quite common to stress the irreconcilable intellectual and scientific differences of opinion between the two scientists (see for example Rieber and Robinson Reference Rieber and Robinson2001). After Wundt abandoned his early theory of perception as unconscious inference, which was strikingly similar to Helmholtz’s, it appeared as if both scientists took radically different intellectual paths. For one thing, the idealist tenets in Wundt's psychological theory – and especially his emphasis on the centrality of volitional processes in accounting for our mental life – are all too often contrasted with Helmholtz's experimental, and according to some even positivist and anti-idealist stance (for a recent example, see Kohls and Benedikter Reference Kohls, Benedikter, Giordano and Gordijn2010). The analysis presented above, however, that points to a definite voluntarist tendency in Helmholtz's thought, could help attenuate this view somewhat, and restore the continuity between the two psychologists. Some decades ago, Leary even hypothesized that Wundt's voluntarism, and his relation to Fichtean philosophy specifically, might even have been mediated to some extent by his contact with Helmholtz, although this remains an altogether tentative hypothesis (Leary Reference Leary1980; also see Richards Reference Richards, Bringman and Tweney1980). It remains, however, that Helmholtz's and Wundt's theories are more in alignment with each other than is commonly assumed, especially when it comes to their emphasis on the central place of the will in their psychological theories. Consequently, the discussion of Helmholtz's indebtedness to Fichte as presented above does not only have a philosophical significance, but could likewise help bridge some important gaps in the historiography of psychology.
