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Rohault, Jacques (1620–1672)


Jacques Rohault was a mechanistic Cartesian experimental physicist. He was born in Amiens, France, and earned his MA in Paris in 1641. There, he became Claude Clerselier's Cartesian disciple and son-in-law. He was Pierre-Sylvain Régis's teacher and converted him to Cartesianism. In the 1650s Rohault was a private tutor in Paris, and his "Cartesian Wednesday" evening lectures, complete with laboratory table demonstrations, were attended by many members of the noble class, women as well as men, and did a great deal toward popularizing Cartesianism. His Traite de physique (Paris, 1671) was a standard text for nearly fifty years. John Clarke and Samuel Clarke, rather than writing a Newtonian physics, translated Rohault's work into Latin (1697) and English (1723) and added Newtonian footnotes to correct Rohault's Cartesian mistakes. The Traite contains descriptions of explanations and experiments in support of Cartesian mechanistic physics. Like René Descartes, Rohault holds that these explanations are only probable because absolute certainty is unattainable by humans.

Also in Paris in 1671 Rohault published his Entretiens sur la philosophie, in which he defends the thesis that Cartesian principles and Christian doctrines do not conflict because each pertains to a separate and distinct realm of truth and knowledge. The book was popular, but Rohault's position was generally viewed as heretical by the Catholic Church.

Rohault opposes Nicolas Malebranche's occasionalism and presents his own mechanistic Cartesianism based on eight axioms he takes to be self-evident:

(1) Nothing (that which has no existence) has no properties

(2) Something cannot possibly be made of nothing, that is, nothing cannot become something

(3) No thing or substance can be annihilated, that is, something cannot be reduced to nothing

(4) Every effect presupposes some cause

(5) If one does not cause an effect, that effect necessarily depends on some other cause

(6) Everything endeavors to continue in the state in which it is (an early Cartesian rendering of a principle of inertia)

(7) Every alteration is made by some external cause, that is, in opposition to Aristotle, no material thing can alter itself through an inner power, force, or form

(8) Every alteration is proportional to the force of the causal agent

Certain propositions follow logically from these axioms, but Rohault says these truths of reason remain purely formal and have no application if there are no existents. Thus, the first task in understanding the world is to seek out existents. In strict Cartesian order one knows first one's own self, whose existence Rohault proves syllogistically:

(a) From principle (1) above, whatever has properties is something

(b) Thinking is a property

(c) Whatever thinks, therefore, exists as something because it has the property of thinking

(d) I think

(e) Therefore, I exist

Reasoning with these principles about ideas and sensations leads to knowledge of the essences of mind, God, and matter and to proofs of the existence of God and of matter. The essence of mind is thought; of God, necessary existence; and of matter, extension. Rohault states that mind and matter are completely different but that God so created the human mind or soul such that motions caused by material impressions on the sense organs and in the brain of the body with which it is united give rise in the soul to sensations and ideas. Neither sensations nor ideas resemble material things, and so resemblance is not necessary for knowledge. It is simply the nature of sensations to give knowledge of the existence of material things, and the nature of some ideas is to give knowledge of the place, situation, distance, magnitude, figure, number, and motion or rest of material things.

Rohault's method in physics is to reason mathematically about experiments before conducting them. His goal is to explain the sensible effects of material things. For this only the primary material properties of size, figure, motion, and arrangement of divisible, impenetrable particles in a plenum are needed; occult qualities such as Aristotelian forms are unnecessary.

In Entretiens de philosophie (Paris, 1671), the companion volume to the Traite, Rohault explains in mechanical terms Cartesian opinions on animal machines and transubstantiation. Animal behavior, he claims, can be explained if animals are completely material; human behavior, however, requires a rational soul that is immaterial, hence indivisible, hence immortal. For Cartesians, the sensible qualities as they exist in material things are not seen, tasted, and so on as one sees and tastes them, but are merely the powers bodies have, determined by the size, figure, motion, and arrangement of their particles, to cause sensations in the mind. There is no further explanation of these powers beyond the fact that God made the correlations between bodily movements and one's sensory experience.

Transubstantiation, then, is the point-by-point replacement of bread and wine by Christ's flesh and blood. Therefore, the flesh and blood of Christ that occupies the places (is bound by the surfaces) formerly occupied by bread and wine causes sensations exactly like those that the bread and wine formerly caused. Consequently, real accidents or Aristotelian forms subsisting separately from Aristotelian matter as postulated in the scholastic explanation of transubstantiation are unnecessary. There are further physical explanations and assurances that Cartesian principles do not contradict Catholic doctrine in Oeuvres posthumes de Rohault (Paris, 1682).

Overall, Rohault disclaims metaphysics and says that although the substitutions are miraculous, even his mechanist explanation of transubstantiation is only a solution to a problem in physics. His work illustrates the strong empiricist stress on observation and experiment toward probable mechanistic explanations in physics so prominent in many Cartesian philosophers. Finally, use of Rohault's Traite as a physics textbook merely with addition of Newtonian footnotes constitutes a major shift to nonmetaphysical, explanatory concerns in science.

See also Aristoteliasm; Aristotle; Cartesianism; Clarke, Samuel; Descartes, René; French Philosophy; Malebranche, Nicholas; Régis, Pierre-Sylvain.


Schmaltz, Tad M. Radical Cartesianism: The French Reception of Descartes. New York: Cambridge University Press, 2002.

Watson, Richard A. The Breakdown of Cartesian Metaphysics. Indianapolis, IN: Hackett, 1998. Originally published as The Downfall of Cartesianism, 16731712: A Study of Epistemological Issues in Late Seventeenth-Century Cartesianism (The Hague: Nijhoff, 1965).

Richard A. Watson (1967, 2005)

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Rohault, Jacques


(b. Amiens, France, 1620; d. Paris, France, 1675)

natural philosophy scientific methodology.

Rohault was the leading advocate and teacher of Descartes’s natural philosophy among the first generation of French Cartesians. Little is known of the details of his life, especially the early years. Most accounts of his career derive from the largely retrospective reports of his father-in-law, Claude Clerselier.1 Rohault was the son of Ambroise Rohault, a wealthy wine merchant, and Antoinette de Ponthieu. He received his early education in Amiens, most likely a scholastic training at the Jesuit collège there.2 He completed his studies in Paris, where, apart from his formal academic routine, he reportedly haunted the shops of artisans, exchanging information on mechanical contrivances and procedures. Rohault also began to teach himself mathematics, eventually gaining enough mastery to establish himself as a private tutor.

The circumstances under which Rohault embraced Cartesianism are far from clear. Alexandre Saverien, an eighteenth-century biographer, claimed that Rohault’s philosophical studies ultimately attracted him to Cartesianism and that he subsequently found his way into the Cartesian circle led by Clerselier, an avocat of the Paris Parlement and later editor of Descartes’s correspondence.3 Clerselier reported that lie had studied mathematics under Rohault (although lie does not date the origin of the relationship),4 and it is therefore possible that Clerselier recruited his young tutor for the Cartesian school. As if to consolidate the association. Rohault married Clerselier’s daughter Genevieve, probably in 1648.5

Rohault’s contemporary fame rested on the very popular weekly lectures he held at his house in Paris, beginning sometime in the mid-1650’s. He rapidly became the leading Cartesian practitioner of the scientific lecture with experimental illustration. His fluent style of lecturing and lucid restatements of Descartes’s physical theories made Cartesian science intelligible to large segments of the educated Parisian public for the first time.6 Since his predominantly lay audience shared the nondogmatic, hypothetical, and experimental ideology of the new science at midcentury, Rohault played down the more dogmatic tendencies of Descartes’s work and sought to join Cartesian explanatory principles to experimental practice and to give a probabilistic interpretation of the truth value of specific explanations.

The lectures covered, one by one, the major problems of natural philosophy. Rohault began each session with a discourse on the general nature of the subjects under discussion, inviting interruptions by questioners and opening the floor to debate at the end of his exposition. Then, starting from the basic mechanical principles of Descartes’s philosophy, he moved to explanations of the particular phenomena under examination, confirming the explanations by experiments. The most famous of the experiments were those on the weight of the air, including a simplified apparatus for performing Roberval’s and Auzout’s demonstrations of “a void within a void”; the production of artificial rainbows; and a meticulously ordered series of experiments illustrating the Cartesian explanation of magnetism. These demonstrations, scheduled in advance for a given subject, attracted large crowds of courtiers, bourgeois, administrative officers, and foreign virtuosos.

During the 1660’s Rohault emerged as the arbiter of Cartesian scientific affairs in Paris. While his lecturing and tutoring continued, he also became an active participant in the Montmor Academy and other circles of leading natural philosophers. In 1665 Rohault recruited Pierre-Sylvain Règis to the Cartesian movement. After several months of instructtion in Cartesian science and the arts of the confèrencier, Régis was sent by Rohault to spread the doctrine in Toulouse. Rohault also organized the ceremonies marking the return of Descartes’s remains to Paris from Stockholm in 1667.

Rohault’s masterwork, the Traité de physique (1671), became the era’s leading textbook on natural philosophy. Intended as an elementary synthesis of Cartesian science, it was largely based on the material and pedagogical approach that Rohault had developed in his confèrences. As Paul Mouy observed, Rohault did not wish to appear as a mere Cartesian partisan in the Traité, but as a sympathetic arbiter between the systems of Aristotle and Descartes.7 Hence, adopting the standard scholastic division of the subject matter of natural philosophy (no doubt to ease acceptance of the work in the schools), He strove to separate the supposed views of Aristotle from the bastardizations of the medieval corjentators.8 The Traité usually presents Aristotle as having been generally correct in his approaches and broad conclusions,9 and often introduces Cartesian views as more complete elaborations of Aristotelian foundations. The revolutionary implications of Cartesian metaphysics and epistemology are somewhat played down.

In the Traité Rohault accepted Descartes’s principles of natural philosophy: that the essence of matter is extension, that the universe is a plenum, that the quantity of motion in the universe is conserved, that three kinds of matter or elements exist, and that the mechanical contact of bodies is the only cause of change of motion. His discussion of the essential topics in Cartesian science—dioptrics, theory of colors, cosmology and vortex celestial mechanics, meteorological phenomena, and mechanistic physiology—follows the Cartesian line as set down in the Principia and in the posthumously published Traité de l’homme. Although Rohault also recounted the Cartesian laws of motion,10 he had surprisingly little to say about the “rules of collision,” which had become a central problem in natural philosophy with the appearance of Descartes’s Principia in 1644. Rohault presented only two rules for cases of inelastic collisions.11 Usually well informed about the latest developments in natural philosophy, he made no mention of the recent work of Wren, Wallis, Huygens, and Borelli on such laws of collision. In addition, despite his calls in the preface to the Traité for a quantitative approach to natural philosophy, Rohault (like Descartes before him) made little use of mathematical argument to establish his positions.

The strength of the Traité and its contemporary appeal lay in Rohault’s ability to weave new experimental findings, as well as his knowledge of craft and chemical processes, within a verbal web of Cartesian mechanistic discourse. Even hostile critics, such as the anti-Cartesian Lagrange, noted that Rohault’s presentations were fuller, more systematic, and better integrated with experiments than comparable sections in the Principia.12 Perhaps the best examples of Rohault’s procedures were his discussions of the experiments concerning the void and his analysis of the nature of liquids.

Recasting most of Pascal’s experiments on the void in Cartesian terms, Rohault maintained, against Pascal and Torricelli, that the space at the top of the Torricellian tube is only apparently void and in fact is filled with Cartesian subtle matter. The space cannot be void, because it manifests the physical properties of transmitting light and responding to changes in temperature by expanding or contracting.13 Nevertheless, Rohault accepted the explanation of the experiments on the void on the basis of a concept of the weight of the superincumbent air.14 In several instances, however, he conflated this concept with the properly Cartesian conception of vortex flows of subtle matter, such as he conceived to be caused by pulling out the plunger of a syringe used to draw air or water.15 This conflation allowed Rohault to claim that when one pulls the plunger of a syringe of which the opposite end is closed, the motion of the plunger squeezes the subtle matter out of the surrounding air, through the pores in the tube, and into the supposedly void space thus created.16

In his chapter on the nature of liquidity, Rohault made use of the putative mechanical properties of the air and of the existence in bodies of pores of various sizes, in order to formulate Cartesian explanations of the behavior of the famed Batavian glass drops, the shape of the meniscus in wetted and unwetted glasses, and some examples of capillary action in narrow tubes.17 He seems to have been the first to observe these capillarity phenomena systematically.18 As in his analysis of the void, Rohault was cognizant of the latest experimental findings and attempted an explanation employing Cartesian terms accompanied by a compelling array of experimental manipulations that seemed to confirm his views. One notes the same mode of presentation even in the rather cursory fourth book of the Traité, where his sketch of Cartesian physiology is enriched by reference to the work of Aselli on the lacteals, Pecquet on the thoracic duct, and Steno on the mechanism of muscle contraction.19

The Traité reflects Rohault’s explicit view that natural philosophical explanations are probable at best and liable to falsification by one experimental counterinstance. For Rohault, an explanation is more probable to the degree that it has been formulated by consideration of fewer properties of the explicandum and that it can be extended to cover new experimental phenomena, which may or may not have been suggested by manipulation of the explanatory schema itself.20 This probabilistic interpretation of scientific explanation is similar in its main lines to the contentions of such important contemporaries as Pascal, Huygens, and Mariotte. Hence, partisan devotion to Descartes did not isolate Rohault from the most sophisticated lines of contemporary thought on scientific method. Indeed, one may view him as having systematized the hints toward a probabilistic interpretation present in the latter portions of Descartes’s Principia, and thus as having remained within the Cartesian school by consistently reorienting its methodological assumptions along lines only vaguely suggested by Descartes.21

The success of the Traité de physique was immediate. A favorable review appeared in the Philosophical Transactions of the Royal Society for 17 April 1671.22 The even more laudatory anonymous reviewer in the Journal des sçavans praised Rohault’s avoidance of metaphysical disputation, his recourse to experimental apparatus, and his familiarity with a variety of useful arts.23 Given Rohault’s sympathetic treatment of the subject matter, neither reviewer had reason to view his Cartesianism as a constricting or dogmatic position.

New editions quickly followed at a pace unprecedented for a textbook of natural philosophy. Within five years three new or revised editions were published in Paris, and two in Amsterdam; by 1730 there had been ten separate publications of subsequent Paris editions or revisions thereof.24 A Latin translation (Geneva, 1674) by Théophile Bonet made possible the use of the Traité as a university text. In 1697 Samuel Clarke, then a young Cambridge B.A. and a confirmed Newtonian, published a new Latin translation, adding notes based on Newton’s views in order to counter Rohault’s Cartesian text. He thus opened the second phase of the history of the Traité.

At first Clarke seems to have been motivated as much by a desire to improve Bonet’s clumsy translation as to undermine the Traité from within by insertion of Newtonian views. In subsequent versions (1702, 1708, 1710) Clarke expanded the notes, widened their subject matter, and sharpened their pronewtonian edge to nearly its final form. With the 1710 edition Clarke was able to utilize large portions of Newton’s Opticks, which he had translated into Latin in 1706.25 This version was the first in which Clarke’s notes systematically refuted the text on most important issues in order to advance the Newtonian world view.26 The last edition of Clarke’s Latin translation appeared at London in 1739.

One should not suppose that Clarke’s Newtonian notes were the sole reason for continued use of the text until the mid-eighteenth century. As a comprehensive natural philosophy Rohault’s Cartesianism was still being taught in colleges and read in lay society. In addition, the availability in one text of the two leading interpretations of natural philosophy no doubt contributed to the longevity of the work.

Rohault’s last years were troubled by a rising political and theological reaction to Cartesianism in France. The growing popular influence of Cartesianism, the scientific wing of which was led by Rohault, elicited both official government repression and private literary attacks.27 Cartesianism was especially suspected of endangering public morals and undermining the tenets of the Catholic faith. Rohault’s last work, Entretiens sur la philosophie (1671), sought to reverse this argument by establishing that only the Cartesian interpretation of the Eucharist, as opposed to the scholastic view, provides an unimpeachable basis for the admission of the real presence as an article of faith. Carrying the argument to the scholastic camp, Rohault went on to claim that Cartesian animal automatism is more conducive to a proper understanding of the human soul than the school teachings that attribute immaterial forms to animate and inanimate entities alike. Nevertheless, the Entretiens did little to dampen anti-Cartesian sentiment, and Rohault was still suspected of heresy by some at the time of his death.

In 1670, at the height of his career, Rohault had obtained the privilège du roi for the publication of a collection of treatises on practical subjects, including elementary arithmetic, mechanics, perspective, and military architecture, in addition to a French translation of the first six books of Euclid’s Elements. The writing of the Traité and the Entretiens, as well as the political difficulties of Cartesianism, delayed completion of the project until Clerselier published Rohault’s Oeuvres posthumes in 1682. Clerselier added an important preface to the work in which he attempted to justify Rohault’s Cartesianism. Like the Traité, the treatises grew out of Rohault’s teaching and testify to the wide range of subjects he covered and to the diversity of his students. His “Traité de méchanique” in the Oeuvres is notable for its Archimedean approach and the avoidance of any systematic attempt to link the science of mechanics with the principles of Cartesian physics.28


1. See Clerselier’s preface to Oeuvres posthumes de M. Rohault (1682), unpaginated; and his preface to vol. II of the correspondence of Descartes (1659), repr. in the Adam and Tannery ed. of the Oeuvres de Descartes, V (1903), 630.

2. Paul Mouy, Le développement de la physique Cartésienne, p. 108.

3. Alexandre Saverien, Histoire de philosophes modernes, VI, Histoire des physiciens (Paris, 1758), 7.

4. Clerselier, preface to Oeuvres posthumes.

5. See Mouy, op. cit., 110.

6. See L. L. Laudan’s intro. to a reprint (New York, 1969) of John Clarke’s 1723 English trans. of the Traité, A System of Natural Philosophy, I, xiii. (Hereafter cited as System according to bk., ch., and para. for those employing other eds. of the Traité.)

7. Mouy, op. cit., 116.

8. See System, pt. I, ch. 27, para. 10, on the scholastics’ invention of intentional species; or pt. I, ch. 7, paras. 10–13, where Aristotle’s conception of matter is differentiated from that of the “Aristotelians” and sympathetically compared with the Cartesian view.

9.Ibid., Rohault’s preface (unpaginated), on Aristotle’s correct contention that there is no void, and also on Aristotle’s supposed use of mechanical considerations in explanation.

10.Ibid., pt. I, ch. 5, para. 8, on conservation of state; ch. 10, para. 13, on conservation of quantity of motion; ch. 11, para. 1, on the law of inertia.

11.Ibid., ch. 11, paras. 5–6.

12. J.-B. Lagrange, Les principes de la philosophie, contre les nouveaux philosophes, I (Paris, 1684), 30–31; cited in Mouy, op. cit., 113–114.

13.System, pt. I, ch. 12, paras. 25–26.

14.Ibid., paras. 17–23.

15.Ibid., para 14. See also ch. 22, para. 69, where Rohault attributes the concave shape of the meniscus in a wetted glass to the difficulty encountered by the air in circulating into and out of the top of the glass in order to depress the water level; and paras. 81–82, where capillarity phenomena in thin tubes are explained by the inability of the air to flow freely into the top of the tube and, hence, to depress the water level as much as usual.

16.Ibid., ch. 12, paras. 8–9.

17.Ibid., ch. 22, paras. 47–54, on the Batavian drops; paras. 68–74 on the meniscus; paras. 81–82 on capillarity phenomena.

18. See Oeuvres posthumes, 594; and Florin Périer’s “Avertissement” to his preface to Pascal’s Traitez de l’équilibre des liqueurs et de la pesanteur de la masse de l’air (1663) in ‘Pascal’s Oeuvres completes …, L. Brunschvicg, P. Boutroux, eds., III (Paris, 1908), 280. As the eds. observe (p. 280, n. 1), Rohault’s observations date from at least 1659.

19.System, pt. IV, ch. 6, para. 2, on Aselli; para. 4 on Pecquet; and ch. 3, para. 6, on Steno.

20.Ibid., pt. I, ch. 3, para. 4.

21. See Descartes, Principes, pt. IV, prin. 204, in Oeuvrcs de Descartes, Adam and Tannery, eds., IX, pt. 2, 322–323; or Latin Principia, ibid., VIII, 327.

22. Philosophical Transactions of the Royal Society, no. 70 (17 Apr. 1671), 2138–2141.

23.Journal des sçavans (22 June 1671), 624–625.

24. For a full summary of the various eds. see M. Hoskin “‘Mining All Within,’ Clarke’s Notes to Rohault’s Traité de physique.”

25.Ibid., 360.

26.Ibid., 361.

27. See Francisque Bouillier, Histoire de la philosophie cartésienne, 3rd ed., I (Paris, 1868), chs. 21, 22.

28. But see prop. XXVI, cor. 1, of the “Traité de méchanique” in Oeuvres posthumes, 568–569, where Rohault attempts to justify the resolution of components of velocity in a collision on the basis of prop. XXVI, which deals with the resolution of static forces involved when a heavy sphere is at rest on two mutually intersecting inclined planes.


I. Original Works. Rohault’s chief work is the Traité de physique (Paris, 1671). Hoskin’s article, cited below, gives a full list of the numerous subsequent eds. The Traité de physique is most accessible in the recent reprint of John Clarke’s original English trans. (London, 1723), A System of Natural Philosophy, 2 vols. (New York, 1969), with an intro. by L. L. Laudan. Other works of Rohault are Entretiens sur la philosophie (Paris, 1671) and Oeuvres posthumes de M. Rohault (Paris, 1682). Mention should also be made of an anonymous “Discours des fièvres” appended to the edition of Descartes’s Traité de la lumière published in 1664 in Paris. Mouy first noted the striking similarity between this treatise and the last chapter of the Traité de physique, in which Rohault presented a corpuscular-mechanical theory of fevers along Cartesian lines (Mouy, op. cit., pp. 65, 126). The order and content of the main arguments of the two works are identical. In addition, the “Avis du libraire au lecteur” states that the “Discours” was first presented by a “philosopher and mathematician” at one of the weekly meetings of savants at the home of M. Montmor. There can be little doubt therefore that this brief text constitutes the first published work of Rohault.

II. Secondary Literature. There is relatively little secondary literature on Rohault. Aside from the reports of Clerselier mentioned in the notes, the following are of most value: A. G. A. Balz, Cartesian Studies (New York, 1951), 28–41; M. Hoskin, “‘Mining All Within,’ Clarke’s Notes to Rohault’s Traité de physique,” in Thomist, 24 (1961), 357–363; Paul Mouy, Le développement de la physique cartèsienne (Paris, 1934), esp. 108–138, containing the most comprehensive available summary and analysis of Rohault’s life and work. On Rohault’s views on methodology and their relation to contemporary currents in French methodological thought, see L. L. Laudan’s unpublished doctoral dissertation, “The Idea of a Physical Theory From Galileo to Newton: Studies in Seventeenth Century Methodology” (Princeton, 1966), ch. 8.

John A. Schuster

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