Rationalism

Leibniz's philosophical system reflects an overarching commitment to the idea that there is a rational order to the universe that can be grasped by human minds. This commitment is manifest both in his more general epistemological and metaphysical views on display in works like the Monadology and in the philosophical theology contained in the Theodicy.

The foundation of Leibniz's philosophy is simple, soul-like substances, monads, which Leibniz believes are the only genuine substances in the universe. His argument rests on the claim that genuine substances must be simple, something he takes to be immediately evident to the human intellect. All other things—bodies, human beings, and animals—may be broken into their component parts; only minds are indivisible, and therefore only minds are substances.

Substantive metaphysical conclusions follow. Leibniz maintains that there may be no genuine causal interaction between substances, because genuine causal interaction would require that substances could be changed from without, thereby contravening their simplicity. Apparent changes are actually internal changes of substances. Because all changes are correlated with the states of all other monads, this "pre-established harmony" guarantees that apparent causal interactions will be grounded in actual changes in monads.

The idea of harmony lies at the heart of Leibniz's metaphysics. There is a preestablished harmony among the changes of substances; there is also a harmony between the order of nature and the order of grace, which ensures that the moral order will be realized. This point emerges clearly in Leibniz's claim that this is the best of all possible worlds. Here Leibniz seeks to defend God's goodness against the apparent visible evidence of all the evil in the world. According to Leibniz, this evil merely seems to tell against God's goodness. We know that God is a perfect being, and we know that a perfect being may act only in the best way. Consequently, the fact that God created this world reveals that it is the best of all possible worlds. The appearances that seem to tell against this are merely appearances, and we may therefore have confidence in the goodness and intelligibility of this world.