This paper argues that Spinoza's notions of “conatus” and “power of acting” are derived by means of generalization from the notions of “force of motion” and “force of determination” that Spinoza discussed in his Principles of Cartesian Philosophy to account for interactions among bodies on the basis of their degrees of contrariety. I argue that in the Ethics, Spinoza's ontology entails that interactions must always be accounted for in terms of degrees of “agreement or disagreement in nature” among interacting things. (...) The notion of “power of acting” is used to express the extent to which a thing's conatus is aided or restrained by external causes on the basis of its degree of agreement or disagreement in nature with them. “Power of acting” generalizes the same approach and method of resolution at the basis of the notion of “force of determination” in order to account for causal interactions not only among the simplest bodies but also among more complex individuals. (shrink)