  In saying that substances are essentially active, Leibniz means that they are endowed with forces. More precisely, according to Leibniz, “the very substance of things consists in a force for acting and being acted upon,” (G IV 508/AG 159) that is, each simple substance is endowed with what Leibniz calls primitive active and passive powers. The idea here again sounds Aristotelian: a substance has a certain essentially active component, the soul or substantial form or first entelechy, and a passive component, primary matter. In Leibniz's mature account, the primitive active force is “an inherent law, impressed by divine decree,” that is, it is the law of unfolding or the law of the series of the simple substance. As he puts it in a letter to De Volder, “I think that it is obvious that primitive forces can be nothing but the internal strivings [tendentia] of simple substances, strivings by means of which they pass from perception to perception in accordance with a certain law of their nature, and at the same time harmonize with one another, representing the same phenomena of the universe in different ways, something that must necessarily arise from a common cause” (G II 275/AG 181). Since simple substances are minds, their modifications are representations or perceptions, and the activity of the simple substance will relate to the change or succession of its perceptions. One way to think of this is that each substance has a unique series of perceptions programmed by God to play in harmony with all other substances, and the internal tendency of a substance to move from perception to perception is its active force, or what Leibniz also calls appetite or appetition.