 In the PCP, Spinoza includes the principle Descartes left out, and offers the demonstration Descartes never attempted. PCP IIp23 states: “When the modes of a body are forced to undergo variation, that variation will always be the least that can be.” The demonstration consists of a concise and exclusive appeal to PCP IIp14, Spinoza's rendering of Descartes' law of inertia, according to which “Each single thing, insofar as it is simple and undivided and is considered only in itself, always perseveres in the same state, as far as it can”. But just as Descartes never explains why the PLMM is true, Spinoza never explains why the principle of inertia supports the PLMM. Moreover, as formulated in PCP IIp14, it seems prima facie inadequate to do so. PCP IIp14 speaks of what happens to a body only as it is considered in itself, simple and undivided, whereas PCP IIp23 speaks of bodies undergoing variation imposed by other bodies, and is not limited to simple and undivided bodies. And even supposing PCP IIp14 relevant to what goes on with bodies when considered as affected by other bodies, it does not give any obvious guidance as to what then occurs. It seems on the face of it quite possible that the least total modal variation commanded by PCP IIp23 might involve a greater variation on the part of each of the colliding bodies than would be consistent with either body in question persevering in its state as far as it can on its own. We are of course told they always remain in the same state “as far as they can.” But for this qualification to be of any relevance, it will obviously have to mean “as far as they can, in the face of the influence of external bodies”; but until we know the content of the collision laws, we are not entitled to say how the influence of external bodies affects a body's inertial tendency. Yet it is just this content we need PCP IIp23 to derive.