 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 nature or essence of the substance. Now according to Leibniz, substances do not causally interact with one another (see the entry  Leibniz on causation).  The changes that they undergo derive solely from their own natures or primitive active force, which consequently determines the whole of its history. Many texts suggest that, for Leibniz, the sufficient reason for any state of a substance is its primitive active force (NE 65–6; T 400/G VI, 354; Mon. 18/G VI, 609–610; G IV, 507; G III, 72).