This paper argues that Wolff’s rejection of Leibnizian monads is rooted in a disagreement concerning the general notion of substance. Briefly, whereas Leibniz defines substance in terms of activity, Wolff retains a broadly scholastic and Cartesian conception of substance as that which per se subsists and sustains accidents. One consequence of this difference is that it leads Wolff to interpret Leibniz’s concept of a constantly striving force as denoting a feature of substance separate from its static powers, and not as (...) their replacement. For Wolff, powers are essential possibilities of acting in subjects suited for independent existence. Force is a further ingredient that provides a reason for the contingent operation of powers. Unlike Leibniz, Wolff conceives force narrowly as a principle of actuality, which he calls the nature of substance, as distinct from its principle of possibility, or essence. (shrink)