Michael Pelczar offers a rare defense of phenomenalism, a metaphysical theory outlined by John Stuart Mill in his study of Sir William Hamilton's philosophy but now largely ignored. Pelczar is enthusiastic about phenomenalism and seeks to restore it to a prominent place in the literature from which it has largely vanished after falling into disrepute about mid-twentieth century. As he remarks, Mill was influenced by traditional idealists such as Leibniz and Berkeley and by Kant's critical idealism, although mention should also be made of the influence of the British tradition of hedonistic associationist psychology as it gradually emerged in the works of empiricists including Locke, Hume, Hartley, James Mill, and Bain. In any case, Mill's phenomenalism is a distinctive metaphysics, which he calls the “psychological theory” and contrasts with Kantian “realism” so-called because Kant posits the reality of noumena or things-in-themselves. Pelczar assumes that noumena have for Kant some power to shape our experience. But Kant is commonly read as maintaining that we do not and cannot know anything about noumena, including whether they play any role in our experience of phenomena.

According to Mill, a physical object such as a table can be re-described as a “permanent possibility of sensation” which, when perceived by a conscious person, interacts with her physical nervous system (which itself requires translation into phenomenalist terms) to produce in her a group of sensations (of extension, shape, color, touch, perhaps pleasure or pain, and so on) appearing more or less simultaneously. The permanent possibility is identified only by the sensations it makes possible, and it exists for as long as does the physical object which it merely re-describes. When not perceived, the possibility continues to exist. It can be perceived again at any time to yield another group of very similar sensations exhibiting the same regularity, although the sensations are not in fact the same ones as before since sensations are fugitive feelings. When the possibility or, in other words, the object is perceived, the sole information received by the conscious subject is the group of simultaneous sensations or perhaps only a part of the group sufficient to call to mind the rest. So the table as perceived just is this group of linked sensations. No other information about it is available such as the nature of any real thing or thing-in-itself that might underlie the sensations or explain how they are made possible.

Mill argues that because they continue to exist when the sensations made possible by them are not being perceived, the permanent possibilities capture all that is meant when reference is made to an external reality whose existence does not depend on our experience of it. He goes on to provide a psychological account as to how the permanent possibilities or physical objects come to be seen as something that transcends sensations altogether, despite the fact that our sensations are the sole information we have about the permanent possibilities or objects. He explains that the mind is prone to leap to the unwarranted conclusion that what is really perceived is an underlying physical substratum that generates the sensations, when in fact we do not and cannot perceive anything of the kind. He does not deny that matter or material substance exists as long as it's identified with permanent possibilities of sensation: “Matter, then, may be defined, a Permanent Possibility of Sensation” (Collected Works, 33 vols., 1963–91, Vol. IX, p. 183). If this definition is accepted, he says “I believe in matter: and so do all Berkeleians” but “I do not [believe in matter] in any other sense than this.” In so saying, he is not endorsing Berkeley's speculation that when sensations are not perceived they reside in the mind of God or speculations attributed to Kant about the power of noumena, including Pelczar's speculation that noumena have the power to generate the rational patterns perceived in the groups of senations made possible by the permanent possibilities. For Mill, the perceived groups of linked sensations are all that is known. Nothing is known about the nature of the permanent possibilities or about how the perceived groups of sensations along with their internal links are made possible. His view thus resembles the common reading of Kant, although he is not committed to the existence of noumena. I'm not concerned to adjudicate between contrasting interpretations of Kant's metaphysics, however, and will accept merely for the sake of argument Pelczar's reading that noumena somehow determine the regularities we experience in our sensations.

Physical systems of objects (like the nervous system), actions (movements of bodies or objects), events, causal processes, states of affairs, and so on are also re-described in phenomenalist terms. For instance, causal processes are spelled out as invariable and unconditional sequences of permanent possibilities of sensation and perceived groups of sensations. For the most part, such a process occurs between permanent possibilities of which we are not presently conscious, and this is an important factor in the tendency of the mind to distinguish physical reality from sensations altogether. But causation is a “topic-neutral” term that applies as much to permanent possibilities and perceptions of sensations as to objects and actions. In short, causal sequences are a regularity in the phenomenalist world just as much as they are in the physical world. So, as Mill argues, the process of a bucket of ice changing overnight into a bucket of water from the heat of a burning fire in the fireplace can be re-described in phenomenalist terms, even if the re-description is complex and unwieldy. None of this provides any information about a real physical substratum or a divine mind alleged to underlie orderly patterns of perceived sensations and to explain why causal sequences occur as they do.

As Mill explains in his System of Logic, methods of induction and ratiocination are needed to infer the regularities of simultaneity and causal order which we experience in our sensations, whether we speak in physical or phenomenalist terms. Reasoning is needed, that is, to discern the links in the groups of linked sensations made possible by a physical object or by an action or by a causal process, in other words, by the relevant permanent possibilities of sensation. Reason enables us to perceive the regularities, and it operates spontaneously and very rapidly to discern simultaneity whereas certain methodological canons facilitate its ability to identify causal sequences. Common slogans for phenomenalism are “Berkeley without God” and “Kant without noumena.”

Pelczar rebuts some leading scholars, including Geoffrey Scarre, John Skorupski, and Andy Hamilton, among others, whom he thinks misunderstand Mill and unfairly attack his phenomenalism. I will leave it to the parties concerned to make their replies. But Pelczar also makes a couple of objections of his own against Mill which are puzzling.

One piece of “unfinished business” is said to arise because Mill allegedly fails to deal adequately with “sensation conditionals,” which Pelczar apparently thinks phenomenalism in particular relies upon. Sensation conditionals involve hypothetical propositions of the form “if p were true, then q would be true.” For example, if she comes upon a permanent possibility of sensation described as an apple at a given time and place and perceives it, then a conscious person would experience a familiar group of simultaneous sensations as of roundness, red surface and white underlying tissue, hardness, and so on; or if she perceives part of the group of simultaneous sensations before biting into the permanent possibility, then she would experience an apple-flavored sensation of taste. It is difficult to understand, however, why such sensation conditionals are thought to pose a special problem for phenomenalism. Hypothetical propositions of exactly the same content apply to the physical object commonly called an apple. Given that the physical description of an object as an “apple” is equivalent to the phenomenalist description of the same piece of matter as a permanent possibility of sensation, and that the only information received from perception of the matter in either case is the relevant group of linked sensations, it cannot be true that phenomenalists face a special problem accounting for sensation conditionals.

Some people make a fuss over the truth values of hypothetical statements but nobody doubts their meaning. More importantly, unless the agent is mistaken in identifying the object or permanent possibility as an apple, the protasis is true and so with certainty in practice is the apodosis. As Mill says, these are certified or guaranteed possibilities: the probability of not experiencing the relevant group of linked sensations is zero if the possibility or object is actually perceived. Common sense tells us that allowance must be made for the fact that an agent's sense of taste may vary with changing conditions, as when she is stoned on drugs or sick with a fever.

Critics in the period 1935–60 such as Stout, Chisholm, and Aaron, who claimed among other things that such variation disproves phenomenalism, seem unable to rid themselves of the notion that the object is a real physical thing whose properties such as taste are either present or not present regardless of noise or imprecision in the sentient individual's nervous system. Although they distance themselves from naive realism, they still claim that phenomenalism cannot capture the looming presence of physical reality which we all supposedly know is there implicitly behind our sensations, and none of them accepts that phenomenalists can for convenience use physical terms interchangeably with phenomenalist terms since the one set of terms merely re-describes the other. These are the sorts of objections which dogged Mill from the start, and which he repels in an appendix inserted into his book on Hamilton.

The second piece of “unfinished business” left by Mill, Pelczar says, is due to his failure to make sufficiently clear what the possibilities of sensation are possibilities for. It is not enough that Mill tells us that the possibilities are for groups of sensations exhibiting discernible regularities of simultaneity and causal order. There is the issue of intersubjective agreement. Pelczar argues that different persons generally agree that they are conscious of the same physical objects and actions whereas such agreement is problematic and requires explanation in the phenomenalist world because each person experiences her own sensations without having access to others’ sensations. But intersubjectivity in the physical world, though we may take it for granted, requires no less explanation. The same question arises when we speak in physical terms: how is it that our perceptions of physical objects and actions are generally congruent, given that these things are nothing but re-descriptions of permanent possibilities of sensation and that perception of them provides no more information than the relevant groups of linked sensations?

The only solution available to Mill or to anyone else for the intersubjectivity problem seems to be the hypothesis that humans have roughly similar nervous systems provided the nervous system is not damaged. Since we do observe that different people generally agree about the sensations they perceive from the same physical object or permanent possibility, an inference to the best explanation is that humans possess some such generic nervous system. Mill supports the inference with respect to the primary qualities of matter such as resistance, extension, figure, and motion: “the sensations which correspond to what are called the Primary Qualities …, when perceived at all, are, as far as we know, the same to all persons and at all times” (p. 187). But different persons do not always exhibit agreement in their perceptions of the secondary qualities such as color, taste, and smell: “The sensations answering to the Secondary Qualities … vary with different persons, and with the temporary sensibility of our organs.” The variation in the sensations answering to the secondary qualities is not due to our having markedly different nervous systems. Rather, it is due to certain contextually variable aspects of the nervous system itself. Our senses of color, taste, and smell allow different persons to experience different sensations, for example, just as they allow any one person to experience different sensations of this sort from the same permanent possibility or physical thing depending on changes in the sensibility of her nervous system under changing conditions.

As indicated earlier, the physical nervous system itself – nerves, spinal column, brain, and so on – must in principle be amenable to re-description in phenomenalist terms. A causal process may explain how certain waves of light and of odor or certain configurations of molecules impact our nervous membranes and cause them to convey certain physical impulses into our brain. But no causal process can account for the emergence of consciousness, whether we speak in physical or phenomenalist terms. We do not and cannot know how objects like tables or apples (those permanent possibilities of sensation) interact with the nervous system (that system of permanent possibilities of sensation) to produce the groups of linked sensations perceived by conscious agents.

The mistaken belief that phenomenalists face a special burden not faced by those who speak in physical terms to account for intersubjectivity leads Pelczar astray, in my view. To handle the special burden, he proposes to construct “an ideal space time” modeled on a scientific view of physical space time. In principle, ideal space time would contain enough ideal observers to occupy the indefinitely many (perhaps infinite) perspectives from which each and every permanent possibility of sensation could be perceived as it evolves through time. Yet even if it can be constructed, such an ideal space time ignores or assumes away the pertinent question for intersubjectivity. The pertinent question is: how is it that different conscious persons when placed in the same position to perceive a given permanent possibility or physical thing generally agree about the group of linked sensations which they experience? Pelczar's ideal space time provides no answer. At best, it simply takes for granted that an answer has already been supplied.

Pelczar goes on to offer two arguments for phenomenalism as he understands it. Both arguments deliberately mimic influential yet contested arguments for materialism. One, which he calls “the regularity argument,” argues that phenomenalism is superior to both realism and Kantianism because it relies on fewer assumptions than the others do to yield the same explanations and predictions of the regularities found in the groups of sensations we perceive. But phenomenalism does not purport to explain why the regularities occur; it merely observes that they accompany the perceived groups of sensations and employs reason to discern them. It seems pointless to compare it with realism and Kantianism since they both presuppose knowledge that phenomenalism denies is available to anyone, namely, knowledge of the categorical properties of a real physical substratum and knowledge of the powers of noumena, respectively. The comparative exercise is also potentially misleading because it lends credence to the delusion that there is some way to pinpoint the source of the regularities that explains why they occur as they do.

Pelczar contributes to this delusion, I think, when he claims that the physical world is what causes the regularities to occur as “objective tendencies” of our experience. The physical world is not known to cause anything that is not already present in the phenomenalist world. There seems to be some subtle form of realism that is infecting his version of phenomenalism. This would account for some odd remarks he occasionally makes, such as the contrast he draws between the visual sensation of red and the red color that is a property of a stop sign. The visual sensation is the only information we have: we do not and cannot know if the stop sign is really a red thing.

The second argument offered by Pelczar is what he calls “the correspondence argument,” which argues that for each and every physical thing there is a corresponding permanent possibility of sensation that makes possible our perception of a group of sensations exhibiting simultaneity or causal order. Even uncertain physical things as in quantum mechanics can be treated as corresponding to uncertain permanent possibilities and groups of linked sensations, despite interpretive disagreements between scientific realists and irrealists as to whether atoms or electrons or other sub-particles even exist. But this general correspondence between physical things and possibilities of sensation is not an argument for phenomenalism. It is properly an implication of it.

In defense of phenomenalism as he understands it, Pelczar makes use of some heavy artillery. To update the treatment of sensation conditionals, he brings in the modern logic of “counterfactual conditionals” or “conditional probabilities”; and to sharpen his idea of an ideal space time that does not yet exist as far as we know but that is in a sense continuous with the actual physical world of our experience, he brings in “possible worlds analysis” and makes some deft moves to come up with a possible ideal world that our actual physical world would most likely belong to if it belonged to any possible ideal world. He argues that phenomenalism postulates that some such possible ideal world exists which is probably nearest to our own physical world conditional on our world belonging to any possible ideal world, where the measure of distance is the extent to which the two worlds exhibit similar regularities as objective tendencies of experience. In his view, this represents an updated and more refined version of Mill's phenomenalism.

But the heavy artillery is unnecessary and un-Millian. It also turns phenomenalism from being a fairly simple doctrine that any intelligent person can appreciate into a complicated doctrine that only pointy-headed professors of philosophy can comprehend or take seriously. In truth, the phenomenalist world is simply the actual physical world of our experience suitably re-described, keeping in mind that the only knowledge we have of our physical world consists of groups of linked sensations. When we perceive physical things, the only information we get is the relevant sense data.

This is not to deny that highly precise space time maps might be constructed to represent the actual physical world of our experience and its phenomenalist world counterpart. They are the same map in substance, with references to physical objects, actions, and causal processes in the one case translated into references to permanent possibilities of sensation in the other, and with emphasis placed on the fact that the only information we perceive in either setting is groups of linked sensations. No further information is known or assumed beyond the permanent possibilities and the sensations we experience; nothing, for instance, about the nature of space-in-itself or of time-in-itself as the essential containers of our experience.

Mill disclaims any such knowledge of space and time as real things that can be perceived behind our sensations. As he puts it, “an entity called Time, and regarded not as a succession of successions, but as something in which the successions take place, I do not and need not postulate” (p. 199, emphasis original). He reduces time to the duration of a succession of causal sequences such as the successions of day and night and of the change of seasons, and argues that space depends on time thus conceived insofar as distance is inferred from the duration and intensity of muscular sensations which are needed to cover it: “the idea of Space is, at bottom, one of time – and … the notion of extension or distance, is that of a motion of the muscles continued for a longer or shorter duration” (p. 223). Sensations of touch and sight are also involved, he says, and visual sensations eventually come to dominate and make us forget the role of the others. The domination by visual sensations has been greatly amplified by technological innovations in transportation and communication since Mill wrote, which enable us to see how far it is from any A to any B and to infer how long it will take to complete the distance by car or plane or rocket. As for real entities of space and time, he is concerned only to debunk assertions that they are known to be infinite. Like his father, he provides a psychological explanation of how the mind is led to imagine “the seeming infinity of Time, as of Space” (p. 199).

There are many additional insights and speculations in Pelczar's thoughtful book which are worth further consideration but I have space and time to mention only a couple of them. First, he notes that Mill allows for “pure mental feelings” that are imagined or experienced in dreams and hallucinations. They admittedly are divorced from the permanent possibilities of sensation that re-describe the physical objects and actions we perceive. Pelczar speculates that he follows Berkeley to find a way to distinguish between veridical and non-veridical experiences. But Berkeley does not explain how we do it and merely indicates that whatever explanation is accepted as reasonable can be adopted by idealists. Mill takes a similar line but he acknowledges that phenomenalism cannot explain our admitted ability to draw the distinction (CW, Vol. XXXI, pp. 166–67, 169). He toys with the Kantian suggestion that the ability might be due to the nature of memory-in-itself but he does not affirm it. Remarkably, demented people often lose the ability.

Mill also acknowledges that, while consciousness is just a general name for all of the sensations and other feelings we experience, phenomenalism cannot explain the notion of a self, which we admittedly have, as a thread that connects our series of feelings, past, present, and future (CW, Vol. IX, p. 194). Nor to my knowledge can any other metaphysical theory give a plausible explanation of the self. When he refers to self-development in works such as On Liberty, then, Mill apparently takes for granted our inexplicable belief and identifies development with improvement of our higher faculties of intellect, reason, imagination, and morality, a point to which I will return.

Second, Pelczar also notes that physical objects have different levels of resolution and that if, say, a table is resolved into a higher level consisting of individual atoms and sub-particles that humans cannot perceive even with powerful microscopes and other tools, perhaps non-human sentient creatures might be able to perceive them and thus experience groups of sensations unlike those which we experience from the table. We can assume that these presumably sub-microscopic conscious agents cannot perceive anything at our level even with powerful aids like telescopes of their own, and we have no idea of their sensations made possible by the atoms, electrons, neutrons, and so forth. Would they see atoms as colored? Would they be able to taste quarks or smell electrons? Would causal events involving these things be perceptible? We can only imagine.

I have not said anything about the associationist psychology which is so fundamental to Mill's phenomenalism, mainly because Pelczar does not discuss it. But an understanding of this psychology is crucial to appreciate the important implications of phenomenalism for Mill's utilitarianism and his doctrine of individual liberty. For instance, an event such as one person directly causing non-consensual harm to another is re-described as one possibility of sensation causing non-consensual sensations of pain to another possibility of sensation. Given that pain is inflicted by means of certain physical actions such as striking a blow with one's fist or spreading malicious lies with one's voice amplified by a megaphone or a broadcasting station to publicly ruin another's reputation, these actions and objects too must be suitably re-described, admittedly a complex and awkward exercise but doable in principle. So, sensations of pain are phenomenalist equivalents to a straightforward empirical notion of harm, which may be called “natural harm.” But at least three features of sensations of pain and pleasure are important to appreciate.

First, these sensations are not isolated feelings floating freely in the air. They are components of perceived groups of linked sensations which are made possible by physical objects and actions and causal sequences that are re-described as permanent possibilities of sensation. While any given sensation of pleasure or pain is like other sensations a fugitive feeling, very similar sensations can be experienced again and again by revisiting the same permanent possibility repeatedly for as long as it endures.

Second, the rough similarity of our nervous systems implies that we experience roughly similar sensations of pleasure and pain, although their intensity may vary depending on the sensitivity of our system. Pleasure might even turn into pain (or vice versa) as conditions change, for example, when its intensity changes or when a person becomes high on drugs, or sick with a fever, or mentally ill with dementia.

Third, and most important, pleasant and painful sensations are like other sensations conveyed into consciousness and apparently memory by the nervous system independently of volition and the higher faculties of intellect, reason, and creative imagination that are powered by the will. True, reason is needed to discern the regularities of simultaneity and of causal order in the groups of sensations. But reason can only operate on the sense data delivered by the nervous system. It works a posteriori on the basis of experience; it cannot create the sensations and does not configure experience in an a priori way as with Kant. Even the groups of linked sensations which we perceive are constructed in part by the higher faculties since reasoning is needed to discern the links of simultaneity – the group of simultaneous sensations that forms the complex idea of an object – and of causal order – the sequence of permanent possibilities and perceptions of sensations that forms the complex idea of a causal process or event. I say complex ideas in contrast to the simple ideas or impressions of individual sensations themselves made in memory.

It follows that sensations of pleasure and pain per se must be kept distinct from ideas of objects and events, desires and aversions, and other products of the higher faculties. So sensations of pain – natural harm – must not be conflated with mere dislikes in which sensations of pain are absent, just as natural benefit must be distinguished from mere desires that lack any roots in sensations of pleasure.

Mill recognizes that there are more complex feelings of pleasure and pain than simple sensations. These more complex feelings are in his view constructed by the higher faculties and the will by “chemically” combining or fusing the sensations, whether present, remembered, or expected, with other ingredients, including ideas of objects and actions and causal processes real and imaginary, passions, instincts like fear, and secondary normative principles with their conflicting proximate ends. Sensations of pleasure and pain are always ingredients of the more complex feelings of enjoyment and misery, and are what give to the complex feelings their pleasant or painful quality.

The rough similarity of our nervous systems does not imply that we have similar sets of higher faculties since those faculties can be improved through exercise and permitted to atrophy through lack of it. Different persons exhibit different levels of self-improvement or individuality, in large part because they exercise their higher faculties to different degrees.

As is well known, Mill claims that there are complex feelings of pleasure which are higher in kind and quality than naked sensations of pleasure. Given that different persons have developed diverse sets of higher faculties, all of which remain fallible, people will form diverse and conflicting complex pleasurable feelings, as illustrated by their conflicting sentiments of justice which he sees as complex feelings in which sensations of pleasure including relief from pain, present, recalled, or expected, are fused with competing secondary principles of justice, their various ingredients, and their distinct proximate ends. Moreover, he argues that a person whose higher faculties are sufficiently developed will form complex moral sentiments in which she takes higher pleasure in fulfilling recognized reasonable duties to others, whereas people of undeveloped faculties remain narrowly selfish and inclined to ignore morality. In his view, society must rely on the first principle of utility to resolve these conflicts so as to maximize general utility, that is, collective benefits net of collective harms. To promote this ultimate goal, he defends the employment of a democratic political system in which people who are “competently acquainted” with the different kinds and qualities of pleasurable feelings have sufficient power to check what they consider abuses by popular majorities or their representatives.

Mill surely recognizes that there are complex feelings of pain and misery but, perhaps surprisingly, he never says that such feelings are worse in kind and quality than the elementary sensations of pain found among their ingredients. This asymmetry – the fact that he defends a doctrine of higher pleasures but does not defend a doctrine of higher pains – may seem worrisome. But he stresses that pain is heterogeneous with pleasure. The higher pleasures doctrine says that some pleasurable feelings are higher in quality as pleasure, that is, in comparison to other kinds of pleasurable feelings. This does not imply anything about the comparison between pleasure and pain. Indeed, the sensation of pain – natural harm – can be so chronic and intense that it overwhelms and ruins all kinds of pleasures. So it makes sense to say that any kind and quality of pleasure, high or low, is offset by sensations of pain, and that every kind and quality of pleasure includes relief from natural harm.

A fair question concerns how the higher faculties themselves can be re-described as permanent possibilities of sensation, given that they make possible higher pleasures which cannot be reduced to fugitive sensations of pleasure. Although he does not address this question, it does not pose a problem for Mill. The higher faculty of reasoning, for example, is a part of the brain that, once developed, can perform inferences and enable the experience of higher pleasures from the performances. It can in principle be re-described as a permanent possibility of sensation that makes possibile such higher pleasures, keeping in mind that these higher pleasures always contain among their ingredients sensations of pleasure, present, remembered, or expected. People who do not develop this higher faculty cannot experience the higher pleasure: they are not “competently acquainted” with it.

Although I have cast doubt on his analysis of phenomenalism and I regret his omitting to discuss the associationist psychology which, for Mill, is at its core, I share Pelczar's enthusiasm for phenomenalism and applaud his aspiration to restore it to prominence. Mill's lucid outline of its simplicity and appeal seems to have inclined some leading scholars to take it seriously well into the twentieth century, including Russell, Carnap, C.I. Lewis, Ayer, and even Isaiah Berlin who in an early article admits that he can barely stop himself from declaring that it is “self-evidently true.” But I would hesitate to call any of these people “neo-Millians” for various reasons beyond the scope of this review. In any case, the tides of intellectual history were against phenomenalism even as Mill wrote. Movements such as Comte's positivism, Watson's behaviorism, the classical pragmatism of James and Dewey, logical positivism which came to embrace the verification principle and operationalism, and even neoclassical economic theory as it evolved, all discounted or ignored introspection of our mental phenomena as unscientific and focused instead on studies of physiology, neuroscience, reflex instincts, and observable behavior. In this light, the rise of materialism, structuralism, cognitive science, and the like can hardly come as a shock, despite their shared neglect of human feelings, moral sentiments, and consciousness in general. It would indeed be wonderful to witness a philosophical revival of phenomenalist metaphysics and epistemology, especially if accompanied by renewed interest in the associationist psychology. But do not hold your breath.