Indifference, Necessity, and Descartes's Derivation of the Laws of Motion

Indifference, Necessity, and Descartes's Derivation of the Laws of Motion BLAKE D. DUTTON WHILEWORKINGON Le Monde, his first comprehensive scientific treatise, Descartes writes the following to Mersenne: "I think that all those to whom God has given the use of this reason have an obligation to employ it principally in the endeavor to know him and to know themselves. This is the task with which I began my studies; and I can say that I would not have been able to discover the foundations of physics if I had not looked for them along that road" (Letter to Mersenne, 163o AT I, 144). ' As the letter makes clear, knowledge of the foundations of Cartesian physics is inextricably linked to the knowledge of God. Unfortunately, Descartes never explains why this is the case, and the relation in which his theistic doctrine stands to his physics remains unspecified. In what follows I wish to clarify that relation by examining some of the problems surrounding Descartes's attempt to locate the metaphysical foundations of his physics in his doctrine of God. I begin my analysis with a discussion of the doctrine of divine indifference and argue that this doctrine is of great importance to any interpretation of the foundations of Cartesian physics, insofar as it provides Descartes with a rationale for dismissing the appeal to final causation in scientific explanation. Insofar as this is the case, it supplies an important piece of his justification for mechanism. I will show, however, that the very reasons which allow this doctrine to be used to underwrite a ' All citations from the works of Descartesare from John Cottingham, Robert Stoothoff, Dugald Murdoch, Anthony Kenny,trs.,ThePhilosophicalWritingsofDescartes,3 vols.(Cambridge: Cambridge UniversityPress, 1985& a99x). I followthe Adam and Tannery pagination. I would like to thank Allen Wood,James Ross, Gary Hatfield and ClayStinson for their valuablecommentson earlydraftSof this paper. [193] t94 JOURNAL OF THE HISTORY OF PHILOSOPHY 34:2 APRIL 1996 mechanistic science appear to undercut what is perhaps the most striking example of Descartes's efforts to ground that science in his theism, viz., his derivation of the laws of motion from the idea of God. Since these laws govern all of the interactions among bodies in the Cartesian universe, if the indifference doctrine truly undercuts their derivation, it would seem that Descartes's dream of establishing proper metaphysical foundations for his physics in his theism fails at a rather crucial point. After laying all of this out I will argue that these tensions can be resolved once we clarify the kind of necessity which Descartes assigns to the laws of motion. In order to do this, I will examine these laws in light of his views on the necessity of the created eternal truths and the impact of the divine indifference doctrine on those views. In this I will challenge the widely held view that the laws of motion are themselves eternal truths, 2 and argue that it is only when we properly differentiate the kind of necessity which pertains to each that we can resolve the apparent conflict between the divine indifference and the derivation of the laws. 1. DIVINE INDIFFERENCE AND THE REJECTION OF FINAL CAUSATION In the replies to Mersenne, Descartes adamantly denies that there is anything in the nature of the things that God has created which could have compelled him to create as he did rather than otherwise. In fact, Descartes asserts, nothing in the nature of things could have even acted as a reason for God to have created them such since the divine will is absolutely indifferent to all which it has created or could create. He writes: "It is self-contradictory to suppose that the will of God was not indifferent from eternity with respect to everything which has happened or will ever happen; for it is impossible to imagine that anything is thought of in the divine intellect as good or true, or worthy of belief or action or omission, prior to the decision of the divine will to make it so. I am not speaking here of temporal priority: I mean that there is not even any priority of order, or nature, or of 'rationally determined...