  IIIp6 introduces, perhaps, a slightly different thesis about what it means to be a particular thing: a particular thing is one which strives to persevere in being. IIIp6’s dependence upon IIIp4 suggests that this thesis means that any object will remain in the same state unless external causes affect it. Such a thesis appears to be a principle of inertia, and, indeed, Spinoza seems to invoke a principle of inertia in the terms he uses at IIIp6. ‘Conatus’ is a technical term of Cartesian physics, referring to an object’s motion. Spinoza himself uses the term in this way in his exposition of Descartes’s Principles of Philosophy. (Compare, for example, Descartes’s Principles II, art. 37, III, art. 56 and III, art. 58 to Spinoza’s exposition IIp14c, IIID3 and IIp17, respectively). Moreover, at IIIp6, in addition to using the term ‘conatus’ again, Spinoza also uses the same phrase that he uses in framing a different principle of inertia at IIp14 of his exposition of Descartes’s physics: “as far as it can by its own power” quantum in se est. This phrase is also one that Descartes himself uses at Principles II, art. 37. (Note however that there is some controversy over how this phrase is to be understood: see Curley’s footnote to IIIp6 in his translation and Garrett, 1999, note 2.) So there is a good textual basis for the conclusion that IIIp6 indeed has this meaning.