Denis Casabianca

1The famous formulation of Charles Bonnet retrospectively and ineluctably classifies Montesquieu in the category of “Newtonian” thinkers: “Newton discovered the laws of the material world: you, sir, have discovered the laws of the intellectual world” (letter of 14 November 1753, Masson, t. III, p. 1478). The idea of a Newton of the human world can also be found in certain readings that stress the formation of a new science in L’Esprit des lois, a science that takes its model from modern physics. The comparison which Montesquieu makes between a monarchy and the “system of the universe” (EL, II, 7; see also Pensées, no. 5) seems to lend credit to this interpretation, yet nothing proves that the passage refers to Newtonian gravitation (Casabianca, 2003, p. 85-86). To decide, we first have to see what is the status of the Newtonian reference in the texts of Montesquieu which are devoted to the sciences.

2Montesquieu a reader of Newton? The spirit of the times was such that a man of letters could not ignore his importance in the Europe of the scientists. In Montesquieu’s notes, Newton appears, with Locke, as the figure of English learning (Spicilège, no. 572a, OC, t. XIII, p. 504). Montesquieu relates an anecdote on the two men, thus echoing their popularity (Pensées, no. 1105). One can then think that his English relations, Nollet’s classes, which are striving to spread in France the work of Newton on light and whom the Academy of Bordeaux invited to give public lectures as early as 1741 (OC, t. XIX, letter 509), or yet again the relations he maintained with abbé Conti, a Newton popularizer, might have given him the opportunity indeed to frequent the work of the English scientist. And in fact, the library at La Brède is well supplied in Newtonian works: we find several editions of the Optics, translated by Coste (Catalogue, no. [‣]: Amsterdam, 1720; Catalogue, no. [‣]: Paris, 1720; Catalogue, no. [‣] and no. [‣]: Paris, 1722), a Latin edition of the Principia (Catalogue, no. [‣]: Amsterdam, 1714), an Abrégé chronologique (Catalogue, no. [‣]), and a collection of texts on the physics, natural religion, and mathematics of Leibniz, Clarke and Newton, where we find part of the correspondence between Leibniz and Clarke (Catalogue, no. [‣]: Amsterdam, 1720). If the presence of these works can be the sign of a certain interest, it is more difficult to determine with any precision what attention Montesquieu gave to them. What did he actually read, and what did he remember of Newton’s physical theses? We must then observe that, contrary to what might have been expected, the references to Newton in Montesquieu’s texts are relatively infrequent: we find three in the scientific writings (one in the Discours sur la transparence des corps, two in the Essai d’observations sur l’histoire naturelle), two in the Pensées and five in the Spicilège (these latter references moreover manifest not a reading of the works themselves, but rather notes for readings that touch on Newton – Voltaire, Dortous de Mairan, Polignac, Castel, Saurin). It is on the basis of these textual indices that we can attempt to determine what Montesquieu effectively remembered of Newton in science, and measure whether the influence is such that it could really have informed the project of L’Esprit des lois.

3While Montesquieu ordinarily makes reference to Descartes to describe the project of a piece of research in physics, he never refers to him where examination of a question of physics in the contemporary debate is at issue. The physics of Descartes is no longer on the agenda, which does not mean that Cartesian physics is abandoned. On the contrary, Newton’s theses on optics are presented as innovative (Alberto Postigliola, p. 104): in 1720, Montesquieu alludes to “Newton’s experiments” on colors (Discours sur la cause de la transparence des corps, OC, t. VIII, p. 240-241); he returns to this theme the following year, when describing a microscopic observation of an insect that “seems to confirm Mr. Newton’s new system of colors” (Essai d’observations sur l’histoire naturelle, ibid., p. 195). Newton’s experiments make it possible to formulate an hypothesis in 1720; his system guides Montesquieu’s observations in 1721: we might then think that he must have clarified his reading of the Optics in the interval, which could be confirmed by the existence of a Memoir on the extract of Newton’s Optics, but the attribution of the manuscript to Montesquieu is not absolutely decisive (ibid., p. 248-257). Moreover, judging by what he remembers in this text (the simple formulation of a thesis), it does not seem necessary to have gone very far in the reading of the work. As for the mathematisation that characterizes Newton’s approach, we find no trace of it in Montesquieu’s texts.

4Therefore these references must be relativized, by stressing the fact that the reference back to Newton at the moment of this 1721 observation does not imply the abandonment of a Cartesian attitude to the benefit of a Newtonian position. Montesquieu is aware of the insufficiency of Descartes’s theses in physics, and draws from this fact the necessity of improving Descartes’s physics. Thus one can remain Cartesian while recognizing Newton’s superiority, and by considering him as one who made great discoveries: “Saurin has rightly said: Mr. Newton’s fine discovery on colors was made by him only because he believed the colors were in objects. Descartes, who was in the true philosophy, and knew that colors were made up only of the modification of matter, would never have made this fine discovery.” (“Saurin dit très bien : La belle découverte de M. de Newton sur les couleurs n’a été faite par lui que parce qu’il croyait que les couleurs étaient dans les objets. Descartes, qui était dans la vraie philosophie, et qui savait que les couleurs ne se faisaient que par la modification de la matière, n’eût jamais fait cette belle découverte.”, Spicilège, no. 591, p. 516). This text lets us think that this discovery must make it possible for “true philosophy” to orient his research. As Fontenelle had pointed out, attraction from afar allows one to account for everything without explaining anything; the “modification of matter” evoked by Montesquieu thus concerns not only the changes in material bodies (which attraction can account for) but modifications by matter. For a Cartesian, it is only the encounter of bodies that makes it possible to explain truly the changes in their motion. Hence, the vacuum assumed by Newton is also suspect, since it supposed actions from afar, in other words phenomena which one cannot explain by modifications of matter (since one suppresses the necessity of contact between bodies).

5Indeed we find a trace of these questions in Montesquieu’s notes concerning Newton. In the Pensées, when Montesquieu refers to Newtonian attraction, it is to poke fun at Dortous de Mairan (Pensées, no. 1320); another fragment Voltaire sharply criticizes Voltaire and his belief in attraction (Pensées, no. 1380). More seriously, the texts in the Spicilège repeat arguments against Newton’s hypotheses, and Montesquieu’s approval for these “good objections” (Spicilège, no. 565, p. 499-500) clearly appears. The arguments bear on the status of space and the problem of infinite divisibility (Spicilège, no. 489a, p. 439), on the examinations of Newton’s arguments against the Cartesian hypothesis of vortices, and on the question of vacuum (Spicilège, no. 565). The texts manifest both an excess of geometrization, which would give a false appearance in the system’s theses, and an abuse of hypotheses, of suppositions that are not examined. The two points are linked: the calculations seem to justify the hypotheses, while the theses established by mathematical demonstration are only the necessary developments; geometry here masks the insufficiencies of the system. Montesquieu is thus here clearly the echo of the critics who organize themselves formally around the excess of geometry and arbitrary suppositions, and who attack the notions of attraction, vacuum and space.

6We see in the academic discourses, where Montesquieu evaluates papers proposed to resolve such and such a problem of physics, that the frameworks of research in which he is inscribed are not renewed and reformed on the basis of the Newtonian discoveries. Alberto Postigliola shows how the questions raised in Discours sur la transparence des corps (‘Discourse on the transparency of bodies’, OC, t. VIII, p. 237-241), where the reference to Newton plays a role, enter “within the problematics of and, so to speak, into an atmosphere which could be defined as ‘Cartesian’ in the broad sense” (“à l’intérieur de problématiques et, pour ainsi dire, dans une atmosphère, que l’on peut définir comme ‘cartésienne’ au sens large”, p. 100-102). On can make the same kind of reading for the Discours sur la cause de la pesanteur des corps (OC, t. VIII, p. 227-234), where one might have expected to see Newton’s physics shaping the debates. But such is not the case. The contributions of Newton are taken into account, since Bouillet’s paper (Jean Ehrard, p. 66-67), which won the contest at the Bordeaux Academy, admits the Newtonian theory of generalized gravitation. But the problem is envisaged in a non-Newtonian paradigm. Vortices remain the hypothesis common to the different papers that are examined; it is the modalities of the vortices’ action that are the object of discussion to maintain this explanatory schema.

7If we compare this text with Voltaire’s “on the system of attraction”, we observe that Montesquieu has not seen in Newton the genius who is able to destroy “without resource all vortices large and small” (Lettres philosophiques, 1734, letter 15), and places himself rather on the side of Saurin and Fontenelle, whose reproaches Voltaire rebuffs. In the conclusion to letter 14, the physics of Descartes is presented as “an attempt”, whereas that of Newton is a “masterpiece”. This text can be confronted with the final passage of the Essai d’observations sur l’histoire naturelle: Montesquieu notes that a simple observation can bring down “systems” of “great geniuses” like Newton, “yet Newton will always be Newton, which is to say the successor of Descartes” (“cependant Newton sera toujours Newton, c’est-à-dire le successeur de Descartes”, OC, t. VIII, p. 223). World systems are the produce of human genius, and are finally but physical fables (Pensées, no. 163). In this sense, neither Descartes nor Newton put an end to scientific research. And Montesquieu is attuned to the inflections brought on by the false appearance given by systems. One detours ever so slightly into vain and dangerous theological debates (Spicilège, no. 489a, p. 439, and especially no. 565, p. 499-500). In this Montesquieu poses as a Cartesian whose purpose is to detach science from all theological influence, precisely at a time when Newtonian cosmology is about to renew and further the argumentation of ecclesiastical science.

8If Montesquieu is interested in Newton, it is through what he allows us to seek, but he is not someone to espouse a system, and does not intend to take Newton as a model. Besides, when he appeals to observation (Essai d’observations sur l’histoire naturelle, OC, t. VIII, p. 212-213; Discours prononcé à la rentrée de l’académie de Bordeaux, ibid., p. 111), it is not to dismiss Cartesian rationalism in science to the benefit of Newtonian empiricism. One must not believe that at this beginning of the 18th century the experimental method is set, that Newton’s work provides the synthesis of this method and speaks with authority. If all the physicists were quite aware of the importance of experiment, the procedures of experimentation were varied, the modalities of experiment and the problem of measurement remained under discussion. Thus Montesquieu can report examples of experiments that are the fruit of non-Cartesian experimental traditions to illustrate the fecundity of the path opened by Descartes, even while certain results challenge those of the Cartesians (LP, [‣]). An attentive reading of the Essai d’observations sur l’histoire naturelle reveals that the narrative form which Montesquieu is trying out derives from a non-Newtonian practice of experiment, and manifests the hesitations and tensions which one can encounter in the discourses on experiment in France at the beginning of the 18th century. We must therefore distinguish the relationship which Montesquieu maintains with Newton’s physicist theses (see Casabianca 2008, p. 94-102) and the use he makes of the idea of necessity in the first book of L’Esprit des lois. In this framework, the reference to rational mechanics, such as it is worked out in Newton’s Principia mathematica, makes it possible to illuminate the reference he makes to the movement of bodies (Charrak, 2013).


Jean Ehrard, L’Idée de nature en France dans la première moitié du XVIIIe siècle (1963), Paris: Albin Michel, 1994.

Alberto Postigliola, “Montesquieu entre Descartes et Newton”, Montesquieu: les années de formation (1689-1720), ed. Catherine Volpilhac-Auger, CM 5, 1999, p. 91-108.

Denis de Casabianca, L’Esprit des lois. Montesquieu, Paris: Ellipses, 2003.

—, De l’étude des sciences à l’esprit des lois, Paris: Honoré Champion, 2008.

André Charrak, “Le sens de la nécessité selon Montesquieu. Essai sur le livre I, chapitre 1 de L’Esprit des lois”, Revue de métaphysique et de morale 75 (2013/1),