One such characterization of a sufficient reason comes from Leibniz’s conviction that in order to preserve the notion of causal activity, without which substances are not really substances—i.e., truly fundamental building blocks of reality—philosophy must revive something like the ancient Aristotelian notion of a substantial form (see the discussion in the entry on Aristotle’s Metaphysics). Each genuine substance, for Leibniz, has what he calls a “primitive active force”. This force is the nat