Ever since the notions of sensibility and irritability were thematized in the works of Francis Glisson (1597–1677), the question of the properties of living matter, and more particularly, the meaning of the vital reactivity embedded in matter has been understood in light of these twin notions and their evolution, from both philosophical and physiological points of view. Guido Giglioni accurately describes how Glisson's legacy deeply marked the eighteenth-century debate, even if he stresses in addition that “Glisson's works witnessed an intriguing development: while the notion of irritability became increasingly popular, the name of Glisson disappeared.”Footnote ¹ In France, the Lockean version of the hypothesis of thinking matter had a better fate. But whether we follow the path of Glisson's thought or that of Locke, it remains the case that the experimentalist approach to living phenomena in Europe in the eighteenth century made room for epistemological debates (how can we know living matter?) and philosophical controversies (what is the nature of matter? how should we understand the link between body and mind?). Accordingly we must take note of this important crossroads between the more “scientifically” oriented thinking dominated by physiologists and the “philosophical” turn of medical doctrines.

In the middle of the eighteenth century, the notions of sensibility and irritability frequently remained confused; their use was sometimes inconsistent or ambiguous among physicians (Steinke Reference Steinke and Mariana Saad2001, 38). The ambiguity and lack of precision in the use of these two terms were constant in the various editions of Diderot and D'Alembert's Encyclopédie. A noted evolution therein can be charted, however, as a consequence of significant alterations by the group of medical associates of this venture. The first edition (1751–1765) showed the marked influence of physicians associated with the Montpellier School, while the Swiss physiologist Albrecht von Haller had an overwhelming influence on the Supplément (1776–1778) and the following editions. Research conducted by John Lough concerning the editors of the first edition of the Encyclopédie makes it possible to estimate the number of associates from Languedoc at roughly around twenty, twelve of whom were doctors at the Medical Faculty of Montpellier (Lough Reference Lough1973, 35). This first team of associates was greatly influenced by various vitalist theses deriving from well-known writings such as those of Théophile de Bordeu and Paul-Joseph Barthez.Footnote ² With the launching of the Supplément and thereafter, the majority of the articles on this subject were written by Haller (Lough credits him with about 200 articles in volumes I to IV of the Supplément). By contrast, only three of Haller's associates were from Montpellier, which explains this evolution.

In the article “Inflammation, Inflammatoires (Maladies),” Jean-Joseph Ménuret de Chambaud (1733–1815)Footnote ³ accounts for the onset of inflammatory phenomena through the untimely stimulation of irritability and not through stagnation of the blood, the mechanistic theory, nor through deficiencies in the preserving soul, the animist theory. Ménuret credits Francis Glisson with putting forward the property of irritability, and suggests as synonyms for this term, sensibility, mobility and contractility, with a reference to the articles “Irritabilité” and “Sensibilité.” In his opinion, this property, which is “totally beyond the scope of the soul,” is present in all organic parts. “It is such that when these parts are irritated, they contract, stiffen, start moving and seem to want to free themselves of that which is irritating them” (Ménuret Reference Ménuret de Chambaud1765b, 8:713b). For Ménuret, this property of living organized bodies is responsible for fevers and irritations when they are subjected to excessive stimulation. Each organic part reacts according to its own vital interest. A perturbing element inopportunely stimulates an organic reaction whose effect is harmful to the organism. The ambiguity is obvious: Ménuret identified sensibility and irritability. Gabriel-François Venel (1723–1775), a physician and chemist who was close to Théophile de Bordeu (Lecornu-Lehman Reference Lecornu-Lehman2006, 54–58), wrote the article “Irritabilité,” which did not dispel the misunderstanding. Irritability is “a term invented by Mr. Glisson and revived in recent times by the famous Mr. Haller to indicate a specific mode of a more general faculty of the organic parts of animals which will be dealt with under the name sensibility” (Venel Reference Venel1765, 8:909b). It was in the article “Sensibilité” by Henri Fouquet (1727–1806), that the elements of the doctrine of sensibility were more specifically developed. “Sensibility is, within the living body, a property that certain parts possess to perceive impressions of external objects, as a result of which they produce movements in proportion to the degree of intensity of this perception” (Fouquet Reference Fouquet1765, 15:38b). This definition reveals the two phases of sensibility, one perceptive, the other active. There is the sensing of the external object and the adapted reaction of living organs. “It is in this double relationship of actions which are so closely linked between themselves that only the imagination can follow them or differentiate between them, that sensitivity should be considered” (ibid.). Feeling allows for discrimination “between useful and harmful physical objects.” Motility brings us nearer or farther away from the object, according to our appreciation of the interest or danger it represents. The attention paid to this subjective dimension is something remarkable, as, from that moment on, the assimilation of the living body to any type of artificial mechanism is excluded. And yet, in that same article, Fouquet wavers regarding the definition of sensibility: there is genuine semantic vagueness. The author speaks in turn of sensibility, a sensory principle or a sensitive soul. Sensibility is the property that enables us to perceive, move or act: glands secrete and excrete as a result of this sensibility. The heart is controlled by this same sensitive principle, just like the workings of the soul. This sensitive principle is the real guarantor of the unity of the organism and is consubstantial with its matter. Fouquet added:

The concepts of irritability and sensibility, therefore, were ambiguous tools. Their use does of course indicate the existence of genuine proximity between researchers, but at the same time, it confronts the historian with a degree of conceptual confusion and misunderstanding when it comes to differentiating systems of thought and assessing reciprocal influences. The debate was carried forward by all the protagonists in a Newtonian perspective, which sought to account for phenomena by exposing laws identifying a property specific to it. “The Newtonian method for all those scientists consists in defining an irreducible property (for example, gravity) and then explaining the phenomenon by writing the law of this property's manifestation” (Huneman Reference Huneman, Wolff and Cipolloni2007, 263). The issue therefore was to know which was the property with manifestations capable of qualifying the living being and with laws that made the living organism scientifically recognizable. Sensibility and irritability were both candidates to become this irreducible property of the living being.

I have chosen here to study the two concepts which indicate the substantial or essential properties of matter in the works of two more or less contemporary scholars who influenced the theoretical reflections of Denis Diderot (1713–1794). Diderot is the philosopher who was most concerned with issues arising from scientific developments in the knowledge of living phenomena. Many scientists and physicians influenced his works and his doctrine; first Bordeu and his montpelliérain colleagues, then Haller, Marat, and Zimmermann. Comparing and contrasting the inquiries of Bordeu and Haller with respect to Diderot's tenets is significant, for it allows us to appraise their influence and to assess their own evolution.

Albrecht von Haller (1708–1777) and Bordeu (1722–1776) studied the properties of living matter and derived from them explanatory principles regarding the workings of organisms. These two physicians influenced Diderot and are mentioned in his work especially when he deals with the sciences of vital phenomena such as physiology, embryology, and medicine (Boury Reference Boury2004, 201–205). Texts such as Pensées sur l'interprétation de la nature (1754), l’Entretien entre d'Alembert et Diderot (1769) and Éléments de physiologie (from 1765) reveal the variations in Diderot's thinking about medicine and physiology. The essential points in the topics dealt with by the philosopher in the Pensées or in the Entretien (the various forms of sensibility and its relevance to movement, the unity of the living being and its different kingdoms) are derived from the ideas of vitalist physicians, especially Bordeu (Kaitaro Reference Kaitaro1997, 40). But the Éléments de physiologie reveals by contrast the impact of Haller's works, notably the Primae Linae Physiologiae (1747), which was translated and edited by Pierre Tarin (1721–1793), a doctor and encyclopédiste. Diderot then begins to diversify his sources, and focused on the relations between sensibility and movement, taking as a basis the biological experiments of Robert Whytt (1714–1766)Footnote ⁴ and Haller, rather than the medical observations of Bordeu. Diderot adopted the positions, experiments, and conclusions of the Swiss physician, notably the famous slogan that the fiber is to physiology what the line is to mathematics.Footnote ⁵ Bordeu and Haller knew each other, and read and criticized each other's work, but the perspectives in which they conducted their research and the objectives that they pursued, diverged.

Albrecht von Haller was an active researcher. An assiduous and systematic experimenter, he contributed to establishing the bases of physiology. He gave few consultations but published numerous medical works. Hubert Steinke has shown extensively and precisely how doctors who, like the Swiss physician, privileged the epistemological over the experimental, practical side, organized their scientific activity. The way the experimental community in Göttingen worked, the articulation between teaching and research, the practical lessons given by Haller, the way he was assisted by Johann-Georg Zimmerman (1728–1793),Footnote ⁶ the laboratory and experimental animals (dogs and rabbits) are all indications of the way in which he conceived his activity (Steinke Reference Steinke2005, 146–153). Like Boerhaave, he remained fundamentally and faithfully a type of mechanicist, improving on the paradigm of Newtonian explanation: general laws explain how animals and men operate and they are the same for all natural bodies. Based on this thesis, Haller studied and experimented on the notion of irritability for which he drew clear limits in relation to sensibility.

Two sources make it possible to give Haller's precise position on irritability and sensibility. In his Göttingen dissertation, De partibus corporis humani sensibilibus et irritabilibus, presented in 1752 and published in English and French translations in 1755, Haller opposes two categories of organic parts: “I call the irritable part of the human body that part which becomes shorter when any foreign body touches it somewhat forcefully; if the exterior touch is steady, the irritability of the fiber becomes all the greater as it gets even shorter, . . . I call the sensitive fiber of man that which, when touched, transmits the impression of this contact to the soul” (Haller Reference Haller1755, 7).

Irritability, the object of a simple quantitative measurement with a graduated ruler, is the criterion making possible a classification of the various organic parts under study, according to the shortening obtained through artificial excitation. Haller made use of an elementary experimental device to draw up an objective classification. Sensibility is described much less precisely, with a definition introducing the notion of soul and impression; it is not the object of any measurement and so an appreciation of its presence and its intensity remains subjective. Measuring the shortening of fibers gives irritability a universal value, as it opens the way to a measurable comparison of the various living tissues. Haller founded his argument on experiments stimulating individual organs. In the case of the individual muscle, Haller attested to the effects of irritability on muscular fibers, their shortening due to excitation, whereas there was neither sensibility nor nerve influence. Any assessment of sensibility remained linked to an individual impression, subject to variations in the conditions of its perception. Irritability and sensibility were not to be confused when characterizing parts. He referred only to experimental criteria and rejected all metaphysical explanation. Nerves, for example, were not irritable because, when applying “a mathematical instrument, divided into small parts, along a nerve of a living dog . . . I irritated the nerve in this condition and it remained perfectly motionless.” Similarly, “the destruction of a nerve which destroys sensibility, does not destroy irritability” (ibid.)

The difference he presented between these two properties, irritability and sensibility, enabled him to specify the extent of his research as well as the metaphysical contradictions of his adversaries. It is not because they receive fragments of the soul, whose sensibility to touch would result in withdrawal to avoid contact that muscular fibers shorten, but through the quality itself of the “gluten” which constitutes the fibers. As the irritable parts are insensitive, they do not come from the sensibility of the soul. Observations and experiments prove that irritability persists after death and in individual parts. For Haller, irritability might be distinct from the soul but did not exclude its existence. He differed from Julien Offray de la Mettrie (1709–1751) who made irritability “the basis of his ungodly and anti-spiritualist scheme” (ibid., 74). Haller rejected any influence of the soul on the vital movements of the individual muscle, by considering irritability “as gravity and attraction are, a property of matter,” in this case of gluten which forms the muscular fiber “without being able to determine the causes” (ibid.). It persists after death and in the individual parts subtracted from the empire of the soul, which for him is unique and indivisible.

The article entitled “Irritabilité” published in 1778 in the Genève-Neufchâtel edition of the Encyclopédie offers another example of Haller's method. In this article, he dispelled all possible confusion as to his own conception: “Irritability is totally different from sensibility; even if it depended on it, it would always be apart, as it is invariably the cause of a shortening which is never the consequence of sensibility” (Haller Reference Haller1778, 19:93). Haller extended the concept of irritability, the simple property of muscular fibers, considering it to be the source of all vital movements. Based on numerous experiments, he endowed it with the expression of an “innate force,” which did not sum up the entirety of vital phenomena. It was associated with a “dead force” and a “nervous force.” The “dead force” corresponded to the material part of bodies – it was a natural physical “contractility” common to bones and cellular tissue found in corpses. The action mode of the “nervous force” resembled that of the soul; this similarity caused confusion between “nervous force” and irritability for a long time. But as the nerve is not irritable, it cannot be a source of irritability, because in that case it would give what it does not itself possess. Irritability, a muscular property, does not require nerves to become an active force: animals without heads, without marrow, without nerves are very irritable. The polyp, which is not known to have a nervous system, as well as anencephalous fetuses are capable of movement when they are subjected to stimuli.

Feeling is an “alteration occurring in the soul when an alteration has occurred in the body” (ibid.), whereas irritability is very active even after death. “Nervous force,” the expression of the conscious will, may add nothing but a “stimulating liquor,” as the “innate force” in willing muscles requires this aid to act, whereas in vital movements, it acts without any help from nerve juices. These conclusions emphasize the difference in nature between sensibility and irritability. Besides these experimental results, Haller used evidence supported by observing the effects of willpower, which varies depending whether the part involved is sensitive or not. On the one hand, the heart, very irritable but not very sensitive, does not obey willpower; on the other hand, the muscles of voluntary movements, highly innervated, are subject to willpower. These findings further linked sensibility to the activity of the soul.

Théophile de Bordeu was first and foremost a practitioner who sought to find rational (scientific, philosophical) solutions to the practical problems his associates were facing and wanted in this way to give medicine bases as solid as those of the rival sciences. His doctrine evolved from Stahlian animism, of which his first two theses, written in 1742 and 1743, bore the mark (Bordeu Reference Bordeu1751), towards a particular vision of the living being, organicism,Footnote ⁷ which retains a certain proximity to materialism as it insists on the material unity of man, and the absence of all recourse to an external principle or an individual substance to account for the functioning of the organism.

The notion of sensibility was one of the central themes in Bordeu's works. Instead of a chain transmitting movement as Descartes had imagined, Bordeu described the organic property transforming physical movement into nervous impulses. “This function of man's which makes it possible to perceive any object, is born within this organ, instrument of the body which is affected in various ways and can be called sensation or feeling” (Bordeu Reference Bordeu1751, § II).

The nerves are animated by a peculiar vital tension which enables them to prepare themselves to perceive the stimulation which is specific to them and so transform this material movement, whatever its origin, into a sensation transmitted to the brain or another organ. In this capacity of the nerve, Bordeu recognized a role in transmitting pain, in regulating the activity of different organs and in being able to know the state of the organs. This function characterizes the living organism and comprehends all the phenomena of contact (material or intellectual) between the living being and its internal and external environment.

Support for this function is constituted by all of the central nervous structures – brain, cerebellum, bone marrow, and the various peripheral nerves. Bordeu emphasized their unity of structure and composition, to account for the homogeneity of function. In his Recherches anatomiques sur la position des glandes et sur leur fonction (1751), Bordeu analyzed the workings of glands. A gland, like eye-sight or hearing, is aroused by a stimulation which is specific to it. Its “appetite” selects elements in the blood, which it separates to constitute its particular humor. It adapts its activity to its needs and to those of the organism. This very appetite is one of the constituents of the particular life of the parts.Footnote ⁸ Glandular secretion is a kind of sensation. The ability to recognize specific stimulations and provide an adapted response characterizes all forms of living beings. Bordeu called this disposition sensibility, associating perception and motion.

Sensibility is a quality of living matter. The doctor observes its effects and deduces existence from it: he does not measure its intensity, nor does he set up its laws. To strengthen his argument, Bordeu emphasized the difference in rhythm and in the intensity of physiological phenomena according to individuals and their condition. Glands function discontinuously; they excrete their humors at particular moments. Bordeu accounted for this functional adaptation through the capacity of glands to perceive changes or influences in other organs, those of the organism as a whole and those of the environment. This property of the glands is a manifestation of sensibility. The adaptability of the glands is impaired by drugs, which affect the nerves: opium numbs the nerves and curbs the activity of certain glands. Bordeu concluded that it was definitely the nerves that distribute this property to the various organs. He justified the difference between the throbbing of living flesh and the cold rigidity of corpses through the vital tension that animates the nerves. Tone is the equivalent of sensibility. If sensibility is the very life of nerves, it appears in the form of permanent tonicity, the source of the capacity to perceive and to act.

The radical difference introduced by the notion of sensibility compared to all the mechanistic modelsFootnote ⁹ is the place devoted to a specifically vital interest. The activity of an automaton is directed to the end for which it is built. The mechanism may be complex but it always fulfills its task in the same way following the program that is imposed on it. The principle of vital reaction lies in the organ's adaptation to the perpetual variation in its conditions of activity as it pursues its own advantage. For the same reason, each part contributes to the general consensus of the body, for each has an interest in the balance produced through the regulated coordination of the activities of them all. This subjective wish to ensure as far as possible its preservation and development differentiates the life of an organ from that of a machine. The basis of this property is to be found in sensibility, considered to be a universal property of the living being. Bordeu endowed sensibility with a determining force in organic workings even if he did not commit himself as to the nature of this organic property. “Would sensibility, in some respects, stem from the essence of a fragment of matter or would it be an attribute essential to its combinations?” (Bordeu Reference Bordeu1751, CVIII).Footnote ¹⁰ The metaphor of a swarm of bees shows the importance he gave to organization, to the consensus of the parts. Sensibility is the principle of organic consensus: each part perceives the activity of the other parts, through sympathy. Modifications in one organ have repercussions on the whole and on the organs that are linked to it in its sector. Each of these organic elements (from the elementary fiber to the most complex gland) possesses sensibility, that is, the capacity to react to stimulation in accordance with its own interest. An alteration in one organ results in an adjustment in the workings of the neighboring organ. Bordeu was not a distant precursor of cybernetics! However, the role he attributed to sensibility exceeds that of a mere quality of perception. He conceived the organism as resulting from particular actions of the organs, whose equilibrium is a permanent adjustment of reciprocal influences. The organism is living because it is sensitive. In the works of Bordeu, the sum of little lives constitutes life in general, because the product of their sum exceeds the addition of their mere juxtaposition. It is as if organic unity results from the united forces of the organs; there is nothing further to explain “the integrative and adaptive order” (Duchesneau Reference Duchesneau1982, 369) which is the end product of Bordeu's theory. Sensibility is not an attractive force of mechanical or physical nature; it is a quality of living matter. The doctor observes its effects and deduces its existence: he neither measures its intensity, nor promulgates its laws. In Bordeu's doctrine, sensibility is not a property opposed to the soul. It is a material property, making it possible to account for the living behavior of matter. Bordeu made no pronouncement as to the existence of a spiritual soul as he had no need of it in his physiological system. If the soul is to govern the body, it must have knowledge of the laws and structure of all the parts. It must be capable of dividing itself among the organs it governs, as they may have poorly regulated or opposing activities. How can it simultaneously be active in the stomach at the time of digestion and at rest in the other organs? Bordeu reserved a place for the immortal and spiritual soul, but with no role to play in the organic working of the human being, and he criticized recourse to a third term, between corporal substance and spiritual substance. His system of sensibility brought together the ability of the living being to react and the possibility that these reactions might be different, even contradictory; this would not be the case for a single and “intelligent” principle.

Based on this radical rejection of a mechanistic conception of organic workings, the Montpellier physicians – following the example of Bordeu – went on to develop an epistemological conception hostile to experiments and experimentation in the field of œconomie animale, the organic working of the body. While remaining cautious about reference to the concept of experiment, a divide is clearly defined between experimenters and observers. Ménuret de Chambaud systematically defended these positions in articles in the Encyclopédie. He was born in Montélimar and defended his thesis in medicine in Montpellier in 1757. Sponsored by Gabriel-François Venel (1723–1775), Ménuret de Chambaud replaced Arnulphe d'Aumont (1720–1782) who resigned from his position in the Encyclopédie. For a long time, no one had acknowledged his participation for what it was worth until the work of Jacques Roger and Roselyne Rey's thesis which attributes around one hundred articles to him, signed “Ménuret,” “M” or “m.” “If the Chevalier de Jaucourt (1704–1780), a true polymath, is excluded, it can be considered that Ménuret's contribution to medicine proper, represents the most important and homogeneous set of articles in the Encyclopaedia” (Roger [Reference Roger1963] 1993, 631; Rey Reference Rey2000, 7). His career was that of a provincial doctor from Montélimar where he practiced. He published several works, including the innovative Traité sur le Pouls in 1768 and L'éloge de Venel in 1777. He made a reputation for himself through his interventions in favor of measures for public hygiene (moving cemeteries to a distance outside urban areas, for instance). In 1784 he settled again in Paris. He studied improvements in therapeutic means and was intensely interested in the reform of medical studies: in 1791, he published l’Essai pour former de Bons Médecins. If he was hopeful at the onset of the French Revolution, he remained a moderate royalist, backing General Dumouriez against the government of the Convention. Condemned to exile, he lost the recognition of his peers, was forgotten and missed out on the place in history he deserved. In addition to his decisive collaboration with the Encyclopédie, Ménuret also wrote articles, “Observateur” and “Observation,” in which he coherently detailed an observational approach that took up a certain number of elements to be found dispersed in Bordeu's works. Observation “is the attention of the soul turned towards the objects that nature offers. Experimentation is this same attention directed to phenomena created by art” (Ménuret 1765, 11:313b), objects “artificially” constructed by experimenting physicists. Ménuret reproached the “laborers of experimentation” with altering nature in their works and with not being able to claim an authenticated knowledge of the phenomenon they were seeking to explain. In this accusation, there was no confusion between experimentation and certain active modalities of exploration.

A comparison between working in a mine and dissecting a corpse shows we have made use of techniques to discover what nature is hiding from us, without modifying either its layout or its structure. The miner who uncovers a vein sees it as nature made it, whereas the chemist who causes the mineral to react to some liquid that dissolves it, is strictly speaking carrying out an experiment, “by which natural results will have been substituted by arbitrary ones” (ibid., 314a). Dissection by an anatomist is nothing more than the revelation of the hidden dispositions of the human body that mere external observation is inadequate to provide, whereas the physiologist who attempts to mix blood with a reactive substance only obtains mediocre, imperfect, “more or less nonexistent” knowledge of his object of study. Of course, one and the other are our only two sources of knowledge, but it is in fact observation which is the most productive of all the sciences. Newton discovered the principle of gravitation through observation, and what about astronomers? Haven't they developed the surest of sciences, without being able to attempt the least experiment because of the distance of the stars?

This recourse to undeniable guarantees only reinforces the scientific value of medicine. If, through observation alone, great scholars have managed to penetrate some of Nature's secrets, doctors can aspire to the same results. Those, however, who favor experimentation have gone astray and are deforming nature rather than helping to get to know it. Choosing this path to knowledge is the consequence of explicit “epistemological” conceptions. Observation is about knowledge; the experimenter's art is about techniques. It is only when observation enables us to understand how nature works that we will be able to attempt to imitate it through experiments. This distinction explains the gap that our doctors emphasize, between living organisms, products of nature which cannot therefore be imitated by human techniques, and machines or experimental artifices which are nothing but unfaithful, delusive copies. The essential originality of their doctrine is brought into play when they favor observation over experiments, since the differences in nature and property between inert matter and living matter (symbolized by sensibility) justifies in their eyes the need for specific treatment, for an autonomous, separate science: a new medical philosophy. “Passing from the physics of inorganic bodies to that of organic bodies, we shall see a reduction in the authority of experimentation and an increase in the empire and utility of observation” (ibid., 315a). Experimenting implies intervening in the work of life and modifying its course. This expedient is unsuited to give us authenticated knowledge of “animal economy.” Observing and analyzing the combination and regulated functioning of the organs on a healthy body as well as on a sick organism, should be the only method for the practitioner to be able to grasp the complexity of living organisms.

The debate between Haller and the Montpellier physicians did not really take place. They were not speaking of the same things; the same words concealed concepts of a different nature. Bordeu made no secret of his goal: to launch the bases of a new medical philosophy. Whereas his condemnation of systems was only a refusal of scientific doctrines imported into the field of life's phenomena, he was not against the rational adoption of their originality. The wealth of his observations, the quality of his philosophical reflections, and the worth of his experience as a practitioner, confer a privileged place to his doctrine in the second half of the eighteenth century. His theoretical formulation, however, remained out of sync with the requirements of a truly scientific discipline. It was a last attempt to give coherence and effectiveness to thought and medical practice which remain broadly “pre-scientific.” On the other hand, Haller's conclusions were less attractive, philosophically speaking, as they mixed experimental results with outmoded mechanistic considerations (particularly the role he attributed to animal minds). Yet his methods and objectives placed him in an experimental context, which was to usher in Claude Bernard's future advances in experimental medicine. Hélène Metzger showed how for Lavoisier in the same period, “an empirical, experimental definition of elements alone was of use if chemistry was to work as a science” (Metzger Reference Metzger1935, 16). The most fruitful instrument in his research method was systematic weighing as a verification criterion for his experiments, just as measuring the shortening of muscular fibers was essential in Albrecht von Haller's work as a physiologist. The sensibility of Bordeu and his colleagues was a philosophical concept likely to attract Diderot. It allows the thinker to grasp the nature of living beings, the unity of the world. In contrast, Haller's irritability may be considered as a scientific concept marking the onset of physiology. The following decades were notable for the continuation of this opposition. The Montpellier school, with Bordeu at the fore, can be considered as the first to attempt a medical anthropology which was to expand into the nineteenth century (Williams Reference Williams1994, 92–93) and for which Pierre-Jean-Georges Cabanis (1757–1808) was an essential link. Sensibility is the property of organs which “enables each one to establish a relationship with the outside world and with itself,” it is the source of man's moral existence, and even if he defends a very personal conception of the workings of sensibility as a fluid, a “moving mass of energy” (Saad Reference Saad2006, 123), Cabanis remained faithful to the Hippocratic, vitalist tradition. Ménuret de Chambaud, Bichat, Cabanis (in France) developed a monist conception in which there was a close link between illness in the organs, psychic symptoms, and moral ailments. The quality in the living being which accounts for this unity is sensibility. The content of the concept varied for half a century but there is no doubt that it was deeply influenced by Bordeu's works.

The eighteenth century was a particularly favorable period in the field of physics, chemistry, geology, and biology. By identifying oxygen, Lavoisier and Priestley epitomized for chemistry what Haller represented with his works on sensibility and irritability. These scholars to whom one can add Charles Bonnet, Wolff, Blumenbach, or Buffon “stimulated new lines of enquiry in the sciences of life” (Engelhardt Reference Engelhardt, Bevilacqua and Fregonese2000, 12), enquiries which they strove to carry forward in a non-metaphysical sense “and in general limited to the empirical sphere of nature” (ibid.). Even if Steinke judged that Haller's immediate successors did not take up the main points of his scientific method, and that it was only with the works of Claude Bernard and Albert Kölliker that his experimental principles and results (revelation of a property in the muscular system independent of the nerves) were revived, developments in physiology show that “Haller's ideal of scientific research finally seems to have been fulfilled” (Steinke Reference Steinke2005, 280–281).

The discussion between physicians concerning two concepts of sensibility and irritability, occurring over a rather short period of time – about twenty years – brings to light tensions in the field of Enlightenment medicine. As Canguilhem put it, “we are in the presence of forerunner signs of an epistemological earthquake” (Canguilhem Reference Canguilhem1988, 17). This debate set into opposition physicians who favored the specificity of living matter and implemented a form of biological holism (Kaitaro Reference Kaitaro1997, 117–123), and those who applied the methods and tools of the natural sciences to living beings, laying the basis of physiology. The evolution of the life sciences will quickly confirm the interest of the methodological reductionism practiced by Haller and his colleagues. Their works provide a firm grounding for scientific and technological medicine. Experimental measuring of the irritability of muscular fibers epitomizes their claims as it is an objectification of a property of the living body. But the objections raised by Bordeu and his colleagues bring out the issues about the possibility of really knowing living organisms and the complexity of their adaptation to the environment and their reciprocal relationships. Their epistemological choices aim to grasp vital phenomena as a whole and their irreducible individuality. Beyond their sizeable contribution to the philosophical discussion, their main interest lies in their participation in the birth of the clinic.Footnote ¹¹