 This divergence over the possible metaphysical grounds for natural laws and collision rules is important for understanding what is fundamentally at stake in another of Spinoza's supplements to what Descartes had himself offered in his Principles of Philosophy. In a letter to Clerselier, Descartes makes clear that of all of his rules of collision “depend on only a single principle, which is that when two bodies collide, and have in them incompatible modes, there must undoubtedly occur some mutation of these modes to make them compatible, but this mutation is always the least possible” (Descartes 1964–74: V, 185, emphasis added). Following Gabbey (1996), call this the “Principle of Least Modal Mutation” (PLMM). Despite is importance, Descartes neither mentions the PLMM is his Principles, nor offers a justification for it in the letter to Clerselier or elsewhere.