1Despite the abundant literature inspired by the remarkable opening chapter of The Spirit of Laws, it seems to us that the meaning of the concept of necessity that organizes this text has not been spelled out clearly.  The virtually axiomatic style of these pages and their rhetorical perfection hide referenced technical decisions about precisely this point, on which statements discussed at greater length are predicated. At first glance, the problem may seem trivial, because it is precisely the discrepancies introduced by human freedom that require all the artificial means that are the subject of the book, that is, the laws established in societies. Yet it is impossible to dispense with a careful investigation of this concept of necessity. Though embedded in metaphysics, it is in Montesquieu’s text that it first manages to win renown as physics, not only because it is necessary to understand from what man departs by exercising his free will, but above all in order to appreciate Montesquieu’s epistemological ambition and, moreover, his project’s place in the philosophy of his time.
2The long-standing ambiguity of the necessity of nomological relations, in our view, contributes to a frequently accepted confusion of two perspectives that do not approach the modern concept of law based on the same relation. In essence, laws can be approached according to the quantity of relationships they express (one would then say that they have this or that level of generality) or according to their modal status. Montesquieu thus decided to assess the nature of their necessity by means of the things which articulate the relationhips??”this is the decision we wish to explain. The importance of the question of the generality of relationships in empiricist thought undoubtedly explains why one would have privileged the first perspective, presuming that a law that was constantly observed would by the same token be necessary and, moreover, that the necessity of the law resided therein: but Montesquieu actually forbears taking such a shortcut, at least in book 1 of The Spirit of Laws. We believe that in this propaedeutic text, he deploys a modal assessment of laws that does not stem from the simple constancy of relations.
3Philological  elements provide useful evidence of the author’s precautions in adopting this orientation. The title of book 1 thus is the object of a series of shifts, which, culminating in the published version, reflects Montesquieu’s desire to avoid stressing the logical quantification of the law: the title initially concerned “universal laws,” then “general laws,” and in the end it became “laws in general”??”“in their most general signification” as the first lines of the chapter specify. Such a change cannot be underestimated: the subject of the study is no longer defined by the globalizing character of certain relations (those of supposedly universal rules) but is extended to all laws as defined by another criterion??”Montesquieu chooses to retain the value of the relationships they express, that is, their modal status.
4The viewpoint proposed in the present study forces us to pay particular attention to the “laws of the physical world”; but it also enables us to resolve some difficulties caused by the very particular importance of this legal register in a text that nonetheless lays the foundation for the study of human laws. Thus we shall find that the prominent role accorded to the laws of matter (that is, as we shall see, to the rules of rational mechanics) does not mean that they provide a model that may be universalized (to which all laws of the animal world or, especially, the deity would be reduced), but rather that they indisputably illustrate to the eighteenth-century reader the modern meaning of necessity in causal relationships. The paradigm is not so much to be found in the quantitative determination of physical laws, which would have to be found in domains not governed by causal relationships, as it is in the basis of their value. At the outset of his book, Montesquieu has us read, not a metaphysical theory of the scale of beings, but an explanation of the need for the scientificity engrained in his time.
5Let us begin precisely with this precise and revealing question instead of reviewing the general organization of the first chapter of The Spirit of Laws.  Extensive debate has revolved around the question of whether this text applies a physical model??”Montesquieu would thereby anticipate the rules of the sociological method that calls for social facts to be regarded as phenomena.  In fact, we shall see in fine that this polemic led Montesquieu to choose as his model a science of effects that, in reality, is not so easily deciphered in the first definitions of The Spirit of Laws. Be that as it may, it appears that relative uncertainty continues to surround essential scientific references in the introductory remarks of the book. It is necessary to distinguish two levels in this passage concerning the modal status of the rules of motion and the general conception of the physical world.
6The theory that motion obeys necessary rules, however banal it might seem, is an excellent illustration of Montesquieu’s purpose, once it is restored to the context in which he takes it up. Leibniz and Malebranche, for different reasons, had not accepted this theory as true, but it is Newton we should think of: not the Newton of the law of attraction over distance,  whom many commentators have projected onto every (vaguely) epistemological discussion among the Enlightenment thinkers, but rather the author of Axioms, or Laws of Motion. It is striking that the concept of necessity expressly claimed in Principia mathematica is equal in rank to that which characterizes mathematical axioms: thus the science of motion approaches pure mathematics, a historic attempt that, this time, should recall the Cartesian innovation. This crucial part of Newton’s masterpiece is the origin of rational mechanics, which triumphed in the eighteenth century. It is appropriate to dwell here on a crucial shift that truly marks the text we are attempting to explain: the meaning of the apodictic mode in this new context is no longer purely logical (the contradictory opposite of which is impossible); it lies now in the thesis that the rules of motion are derived analytically from the concept of bodies (Montesquieu cites mass and velocity in this sense). And we should remember that at the moment when The Spirit of Laws appeared, no better illustration of a concept of necessity as a relationship extrapolated from the nature of things was possible.  From the perspective of the history of ideas, it is important to stress that the idea of necessity without the rest of rational mechanics spread in the eighteenth century precisely among authors who, as far as physics in general are concerned, reduced the laws of nature to the rank of provisional inductive generalizations??”we might cite Buffon, for example:
[…] rational mechanics itself is a mathematical and abstract science […]. Insofar as the demonstration of mechanical effects, such as the power of levers and pulleys, the equilibrium of solids and fluids, the effect of inclined planes, of centrifugal forces, etc., belongs entirely to mathematics and may be grasped by the eyes of the mind with complete clarity, it seems to me superfluous to represent it to the eyes of the body. 
8Thus Montesquieu can assume, by mentioning the rules of motion, an idea of necessity from rational mechanics that his readers will easily accept??”a form of necessity that moreover conforms to the definition of the law he gave at the outset.
9How does this apply to the world of bodies considered as a whole? Before turning to the modes of divine action, it seems permissible to highlight echoes in this text of a conception of the world as a physical system??”by which I mean that the changes that take place in it happen necessarily according to laws that guarantee the permanence of the order that produces them. Without this formal framework, it is impossible to speak of a world in the precise meaning of the term. One might even recognize an allusion to the essential thesis of the global invariance of efficiency (which can be measured according to quantity of motion or by quantity of action) in physical systems according to the notion that “every change is constant”; while it is true that nothing is said about the quantity that is thus conserved, this element is the basis for a second important conclusion. One could imagine it changing in another world provided that proportionality is maintained in one way or another (up to strict equivalence) between cause and effect, which characterizes all interaction in bodies in conformity with the laws. Moreover, although when reading Montesquieu it is highly pertinent to think of the Cartesian cosmogenetic fiction, this is not the place where that reference proves useful.
10The key point, as studied long ago by D. de Casabianca in his book, is what Montesquieu affirms in lapidary fashion regarding divine participation: “God is related to the universe, as Creator and Preserver; the laws by which He created all things are those by which He preserves them.” Two elements, however, seem to me to require further specification: on the one hand, even if the Cartesian origin of this thesis is recognized, it seems rather that Malebranche is evoked here. Aside from the fact that it is never easy to deploy the usual modal categories to account for the principles of Descartes’ physics, the last book of Malebranche’s Recherche de la vérité makes all the consequences that can be derived from identification of the laws of creation and the laws of conservation the ideal field for the method. Before Montesquieu, Malebranche elucidated a theory of the subordination of the structural stability of the world by reference to the nomological similarity between creation and conservation:
[…] it is evident to all who examine things with careful attention that God did not all of a sudden arrange his work in the way it would be arranged over time, that the entire order of nature was overturned, since the laws of conservation are contrary to the order of the initial creation. 
12Far from being secondary, this has major importance in two regards: 1) for comprehending the concept of necessity utilized by Montesquieu; and 2) for assessing his undertaking more generally.
131. No statement has caused as many misunderstandings than that with which the author of The Spirit of Laws characterizes the relationship of God to the material world: “Thus the creation, which seems an arbitrary act, supposes laws as invariable as those of the fatality of the Atheists.” This is where some have hastily spoken of “absolute necessity” without regard for the letter of the text or the literal meaning of this rejection of fatality. We shall come back to Montesquieu’s response to the Jesuits’ attack on this statement, but it is first necessary to grasp what it does and does not say. What Montesquieu says patently concerns the creation of a world, that is, a creation such that what it brings into existence can be conserved??”it is such a notion of permanence, as we have seen, on which the concept of world used in this first chapter is based. In other words, creation is envisioned from the point of view that it continues in a state of conservation without need of supplementary laws: it would not make sense to talk of invariability otherwise. The passage subsequently dispels all ambiguity and shows that it is not a matter of speculating about the laws chosen by the deity, for example, which are allusively related to wisdom (they thus cannot be reduced to brute necessity). The crucial thesis that interests us here is based entirely on the relationship of God to the world, which implies the stability of creation: “It would be absurd to say that the Creator might govern the world without those rules, since without them it could not subsist.” At most, one should infer from the comparison to the fatality of the atheists the exclusion of all extraordinary participation, that is, miracles. Let us now turn to what the text does not state: nothing is said about the principles that might organize the divine intellect and attribute this or that possible world to it, nor about the glory to be had from the fact of bringing it into existence. We understand by the same token that pure possibility and thus metaphysical necessity or contingency are passed over in silence: the laws of the deity per se are set to one side, but they are not subordinated to those of ordinary participation. The need for this kind of mechanism does not exhaust the levels of higher legality, but it focalizes them on the only point of view that matters to Montesquieu: that of the phenomena of the world. One may object that later in this chapter possible laws are mentioned: we shall address this issue, but it is not absolutely relevant to understanding the basis for the necessary laws of matter.
142. Both Descartes and Malebranche the laws of creation are identified with the laws of conservation so as to provide a basis for the cosmogenetic account??”the fiction of the world (to us, the most instructive commentaries on this fiction seem to echo the beginning of The Spirit of Laws to some extent). As stated above, this fiction does not appear to give implicit support for the hypothesis of other worlds, which should simply emphasize their organization according to constant laws. On the other hand, the mechanistic cosmogony is explicit in the statement that “that the world [is] formed by the motion of matter.” What is the point of restoring this reference? Perhaps it is in this seemingly marginal way that the development of the necessity of the rules of motion proves to be the most promising for the general undertaking of The Spirit of Laws. Essentially, the updated cosmogenesis of classical science interested the authors of the eighteenth century in two ways: on the one hand, it could demonstrate the productivity of the laws from which all the varied effects of the world that we see before our eyes derive; on the other hand, it illustrates the advantage of a diachronic exposition that makes the complex order of phenomena intelligible. I would stress that these hypotheses do not prescribe any exclusive scientific approach and, in particular, their deductive appearance, even in Descartes, is accompanied by the necessary recourse to experience. Aside from its various inner manifestations, the cosmogenetic paradigm gave the authors of the Enlightenment a form of legitimacy, if not a model, for understanding man. We recall that Rousseau refers to it in explicit, technical fashion at the beginning of the second Discourse to justify his own hypotheses about the state of nature.  Be that as it may, the fact that Montesquieu cites this model quite naturally in the first chapter of The Spirit of Laws provides a substantial basis for the ambitious claim made in the preface of the work: “I have laid down the first principles, and have found that the particular cases apply naturally to them; that the histories of all nations are only consequences of them”??”it being understood that the examination of facts can only inspire such an approach. What I am trying to suggest here and what I develop fully in a forthcoming book is that, whatever one thinks of it superficially, the empiricist reception of the holistic ambition of Mechanism??”which Montesquieu subsumes under the rubric of the necessity of the material world??”is one of the foremost vehicles for the historical understanding of human phenomena in the Age of Enlightenment: it contributes to the promotion of a genealogy of the datum, expounded according to laws. 
15Although these propositions are generated from the meaning and significance of mechanistic necessity, which Montesquieu places at the heart of his theory of the relations of laws, it is important to note that this reference does not constitute the sole methodological approach at the beginning of The Spirit of Laws. When Montesquieu transitions to the laws of men, he indeed mentions “possible laws” overhanging positive laws: the disjunction is startling and seems to represent a return to more traditional concepts??”the validity of the law no longer seems to be tied to its derivation from a certain nature, but rather is referred to a logical conception of the method. Montesquieu is so well aware of the dichotomy we have in view that he makes very explicit use of it in the polemics over the first chapter of The Spirit of Laws. We shall return to this. But the point of the text itself is not strictly to return to a theory of possibility that was, as we have seen, set aside with respect to God. On the contrary, it shows that the laws established by intelligent beings, which seem to depend on their free will rather than on their necessary nature (both in their invention and in the expressed adherence to them), in reality depend on the same conception of law that was enunciated at the outset. This is also the reason why, even though they obviously exhibit a contingent character (since strictly speaking other laws are possible), as laws, they possess the same structure as the geometric properties of shapes:
Before there were intelligent beings, they were possible; they had therefore possible relations, and consequently possible laws. Before laws were made, there were relations of possible justice. To say that there is nothing just or unjust, but what is commanded or forbidden by positive laws, is the same as saying that, before the describing of a circle, all the radii were not equal.
17Of course, contra Hobbes, human laws cannot be reduced to purely positive entities, because there exist possible relations of justice that precede and normalize them ??”Montesquieu’s position here is sufficiently well-known that we need not dwell on it in the present study. But the fact that the laws established by spirits and men do not exclude counterfactuals thus does not affect the intelligibility of everything defined by book 1, which postulates the necessity of causal relationships??”in the sense that such a postulate is necessary to entertain the possibility of well-founded knowledge of different legal registers.
18Only when responding to the Jesuits’ attacks on the empire of physical laws does Montesquieu, out of necessity, claim that the discrepancy between possible and established laws prevents anyone from accusing him of strict necessitarianism. It is necessary here to review the invariable rules of creation that the author of Nouvelles ecclésiastiques attributes to divine choice, which is precisely what Montesquieu was not studying:
If creation appears to be an arbitrary act, and may it not be so; if God is needed to create; if all beings have such a necessary relationship with him that he could not refrain from creating them, and from creating them such as they are: then the world is necessary like God himself, and the author has reason to maintain that creation presupposes rules that are as invariable as the fatality of the atheists. 
20Montesquieu’s response would consist in applying to the laws of the physical world statements relevant to the laws of intelligent beings. In the Defense, the current necessity of the “law-relations” is compensated by reference to “possible laws” quickly mentioned in The Spirit of Laws??”the two kinds remain distinct (contra Spinoza): “The critic has heard tell that Spinoza admitted a blind and necessary principle that governed the universe: he needed nothing further: as soon as he finds the necessary word, it is Spinozism,” but
[i]n answer to this, the Author has established, in the first place, that there were laws of justice and equity before the establishment of positive Laws: he has proved, that all beings have Laws; that, even before their creation, they had possible Laws; that God himself has Laws, that is, Laws which he himself has made. 
22In a very traditional way, contingency is thus guaranteed on a general level by postulating a realm of possibilities that precede the established laws of the world.
23One should not be mistaken about the significance, within Montesquieu’s project, of this argument produced a parte post in response to outside critics. Again, it is necessary to understand that the author of The Spirit of Laws, perhaps more so than his readers today, is very well aware of the categories he is manipulating??”hence he can take advantage of the conditions of polemic to demarcate space for his theory in the metaphysical discussions broached in the Defense. Thus, when he responds to criticism of the famous comparison of the necessity of relations of laws and “the fatality of the atheists,” Montesquieu returns to the mechanistic paradigm he deployed. He does not simply pretend that he was not talking about the laws of motion; as we have seen, his statement has more general significance, bearing on the meaning of necessity itself??”the author of Nouvelles ecclésiastiques here makes no mistake about it. But Montesquieu’s argument enables him crucially to specify to what he is applying his method: just as in the modern science of motion, it is necessary to establish true propositions with respect to the relations of effects and not make statements about their causal roots:
[…] the Author cannot be made to speak of any other subject but that he is treating of. He is not treating of causes, nor does he compare causes: but he treats of effects, and compares effects. The whole article, that which precedes it, and that which follows, shew that he is here only treating of the rules of motion, which the Author asserts are established by God. He says, that these rules are invariable; and all natural philosophy says so too. They are invariable, because God has resolved that they should be so, and because he has determined to preserve the world. 
25The study of necessary relations thus is nothing less than metaphysics, for the necessity of laws is postulated so as to devise a science of effects??”that is, quite simply, so as to give scientific expression to the cases of regularity that experience makes plain.
26The decision to define law as a necessary relationship inspires two quite distinct theoretical developments: it potentially contributes to the introduction of a new conception of law, several manifestations of which are provided by the reception of The Spirit of Laws. But, provided one attributes real significance to this move, it also presumes a new idea of necessity: the meaning of the apodictic mode is not based on its logical signification, but on the idea that certain relationships may be derived strictly from natures that are subjects of science. Montesquieu utilizes the logical aspect of necessity only for argumentative or polemical ends, and it is ultimately well-known how Kant would ridicule the insignificance of this approach for seriously understanding the modal status of scientific laws.  It is clear that the definition of necessary law posited by Montesquieu should, in the eyes of his contemporaries, be illustrated most clearly by mechanics, and it is hardly surprising for that matter that the relations deduced from the mathematical concept of bodies serve as guidelines for assessing other laws discussed in the book. It obviously is not the point to arrive at this definition in every area, but it shows that, up to that maximum level of determination, the intelligibility of laws derives entirely from the very things to which human reason is applied. But this reference is not simply a case of exemplary understanding of the validity of law-relations: in actuality, everything converges in Montesquieu’s project, in spite of the discrepancies that mark the advent of human freedom, and we should not fail to note the fact that on the threshold of two centuries, the historicization of the datum finds one of its foremost illustrations in the boldest ambitions of Mechanism, that is, in the cosmogonic program, within an empiricism inherited from Locke.
I wish to express my reservations about using the phrase “absolute necessity” with respect to the first chapter of The Spirit of Laws??”this ellipsis ultimately results in a pure and simple identification of necessity in Montesquieu with “fatality” (mentioned only by way of comparison), that is, to be precise, with a notion of necessity that does not essentially presume determination according to laws: which, after all, is outrageous.
Original French quotations from The Spirit of the Laws are taken from the documents prepared for publication that the editors of Montesquieu’s Œuvres complètes have generously entrusted to me: I wish to express my gratitude to them. In anticipation of the appearance of vols. 5 and 6 of the new edition of Montesquieu’s Œuvres complètes, edited by Catherine Volpilhac-Auger (Presses de l’ENS de Lyon, forthcoming 2016), it does not seem useful to give provisional page numbers, since the texts can easily be found in all current editions. [English translation: The Spirit of Laws, trans. Thomas Nugent, ed. J. V. Prichard (London: Bell, 1914), accessed on May 19, 2014, http://www.constitution.org/cm/sol-02.htm.]
Its argumentative structure has been discussed point by point by Jean Goldzink, “Au seuil de L’Esprit des lois. L’audace et le voile,” Revue de Métaphysique et de Morale 4 (2003): 437–461. For an internalist presentation of the structure of book 1, chap. 1, see also Bertrand Binoche, Introduction à la lecture de L’Esprit des lois de Montesquieu (Paris: Presses universitaires de France, 1998), 35 ff.
See Giorgio Lanaro, “Auguste Comte e la genesi dell’interpretazione ‘sociologica’ di Montesquieu,” in Montesquieu e i suoi interpreti, ed. Domenico Felice, vol. 2 (Pisa, Italy: Edizioni ETS, 2005), 551 ff. Denis de Casabianca puts the epistemological findings of sociological readings of Montesquieu into perspective: Denis de Casabianca, Montesquieu: De l’étude des sciences à l’esprit des lois (Paris: Champion, 2008), 125 ff.
Althusser privileges this side of the reference to Newton (Louis Althusser, Montesquieu: La politique et l’histoire, 6th ed. (Paris: Presses universitaires de France, 1985), 32), but relies on the Défense de L’Esprit des lois, in this case in a relevant manner.
Recent studies have expressed some reluctance to speak of a “Newtonian paradigm” (de Casabianca, Montesquieu, 151) with respect to the laws of motion. This hesitation should be offset by the frequent confusion of what, in Newton, belongs to the axiomata seu leges motus, on the one hand, and the law of attraction on the other, which is obviously contingent for the vast majority of Enlightenment thinkers. The moment one understands that this is rational mechanics, it ceases to be any more difficult to recognize an allusion by Montesquieu to scientific advances of his time than it is to situate them??”self-evidently??”in the wake of the Cartesian invention of the rules of collision.
Buffon, “Discours sur la manière d’étudier l’histoire naturelle,” in Œuvres, ed. Cédric Crémière and Stéphane Schmitt, Bibliothèque de la Pléiade (Paris: Gallimard, 2007), 64.
Malebranche, Recherche de la vérité, bk. 6, part 2, chap. 4, in Œuvres, vol. 1, ed. Germain Malbreil and Geneviève Rodis-Lewis, Bibliothèque de la Pléiade (Paris: Gallimard, 1979), 671.
“The investigations we may enter into, in treating this subject, must not be considered as historical truths, but only as mere conditional and hypothetical reasonings, rather calculated to explain the nature of things, than to ascertain their actual origin; just like the hypotheses which our physicists daily form respecting the formation of the world” (Rousseau, Discourse on the Origin of Inequality; this citation is taken from the edition On the Origin of Inequality, trans. George D. H. Cole (New York: Cosimo, 2005), 24). For references to the legacy of mechanist cosmogonies and, first of all, to Descartes’ Principes de la philosophie (especially III, art. 45), see the introduction to my study Rousseau: De l’empirisme à l’expérience, Bibliothèque d’histoire de la philosophie (Paris: Vrin, 2012).
The analogy between mechanist cosmogony and empiricist genealogy is emphasized starting in the eighteenth century; besides Rousseau, one might quote Mérian: “Descartes said: ‘Give me matter and motion, and I will make a world.’ ‘Give me sensation,’ says Condillac, ‘and I will make a man.’” (Johann Bernhard Mérian, “Parallèle de deux principes de psychologie,” Histoire de l’Académie Royale des Sciences et Belles Lettres de Berlin (1757): 382).
Rousseau’s position on this subject is more complicated, because he holds that the relations of justice intended by God are impossible to represent prior to the establishment of laws??”hence the crucial statement that has sometimes been considered legal positivism: “[…] law is anterior to justice, not justice to the law” (Geneva MS, book 2, chap. 4; The Political Writings of Jean-Jacques Rousseau, ed. C. E. Vaughan (Cambridge, UK: Cambridge University Press, 1915), 494.
Nouvelles ecclésiastiques, Oct. 9, 1649, 161–62.
Montesquieu, Défense de L’Esprit des lois, in Œuvres complètes, ed. Daniel Oster (Paris: Seuil, 1964), 809 [The Complete Works of M. de Montesquieu, vol. 4 (London: T. Evans, 1777), accessed May 19, 2014, http://oll.libertyfund.org/titles/864#Montesquieu_0171-04_546].
Montesquieu, Défense, 809.
Immanuel Kant, “Die Postulate des empirischen Denkens überhaupt,” in Kritik der reinen Vernunft, bk 2., “Analytik der Grundsätze”; see especially Kants gesammelte Schriften, ed. Akademie der Wissenschaften zu Berlin, vol. 3 (Berlin: Akademie der Wissenschaften, 1911), 195.