  Apart from the question of how a principle of inertia can give us an understanding of human nature, this interpretation of IIIp6 raises a difficult question about Spinoza’s use of the striving doctrine.  One might object that IIIp6, understood as a restatement of a principle of inertia, extends a physical principle to mind without sufficient clarity. In stating their versions of a principle of inertia in physics, both Descartes and Spinoza are careful to limit the claim to a claim about bodies. Spinoza, for example, in his definition of conatus ad motum, IIId3 of his exposition of Descartes, writes: