 How, then, might inertia be understood so that it supports the PLMM? In good rationalist fashion, we should expect to come to a sound understanding of the principle of inertia by attending to the grounds from which it follows. But just as in the case of the Cartesian conservation law, Spinoza cannot himself have accepted the Cartesian strategy of grounding inertia in the immutability of God's will. Spinoza offers his own principle of inertia in the so-called “physical interlude” of the Ethics, at IIp13 L3C: “a body in motion moves until it is determined by another body to rest; and a body at rest also remains at rest until it is determined to motion by another”. Rather than appealing to divine will, Spinoza's demonstration of this principle seems to proceed from causal rationalism alone.