  The second part of Leibniz's critique of the Cartesian doctrine of corporeal substance relates to the notion of activity. According to Leibniz, substances are not only essentially unities, but also active. As he says in the opening line of the Principles of Nature and Grace: “A Substance is a being capable of action” (G VI 598/AG 207). But Cartesian corporeal substance, insofar as its essence is extension, cannot be itself a source of activity. (G IV 510/AG 161) There are at least two strands to Leibniz's argument on this point. First, Leibniz holds that this is so because he adheres to the classical and Scholastic idea that actions pertain to supposita; that is, only something that can be the subject of predication can be active, and only true unities can be genuine subjects of predication (and not mere phenomena). Put differently, Cartesian extended stuff cannot, insofar as it is infinitely divisible, constitute a suppositum, or subject of predication. But, second, Leibniz believes that something is active if and only if the source of its activity can arise within itself, that is, if and only if its activity arises spontaneously from within itself. This is another reason, then, that individual substances will be understood as mind-like, for Leibniz believes that only minds or mind-like things can originate and alter their modifications.