Like Descartes and Spinoza, Leibniz (1646–1716) was one of the great rationalists. As with his two rationalist predecessors, he believed that reality is knowable by reason. Leibniz's philosophical system is founded on a small number of basic principles, of which the best known are the principle of sufficient reason and the principle of the indiscernibility of identicals (sometimes known as Leibniz's Law).

The first says that there is a reason that every fact is what it is and not otherwise; nothing happens without a reason. For example, Leibniz argued that because there could be no reason for the world to be created at one moment rather than another, the world couldn't have been created at a particular moment. The second says that if two things are identical, they have all their properties in common.

The Life of Wilhelm Leibniz

Leibniz was a polymath almost without peer. He was a mathematician, jurist, historian, scientist, diplomat, poet, inventor, and courtier. His father was a professor of philosophy at Leipzig University, but Leibniz turned down an academic career. He entered into the employ of Baron Boineburg in Frankfurt, while also continuing his study in the law and pursuing his interest in physics, especially of motion.

His best-known works are the Discourse on Metaphysics (1686), the New Essays Concerning the Human Understanding (1704), the Theodicy (1710), and the Monadology (1714). Only the Theodicy was published in his lifetime. Every bit as famous was the series of letters he exchanged with Antoine Arnaud concerning freedom and the concept of an individual, and with Clarke concerning the Newtonian universe. No listing of his writings, even if it were comprehensive, can indicate the scope of his interests, abilities, inventiveness, sheer intellectual power, and prodigality. The task of compiling a complete edition of his work did not being until 1923 and is not yet finished.

Monads and God

Descartes was a dualist, arguing for the existence of two kinds of substance, mind and matter. Spinoza was a monist, maintaining that there was just one substance and modes of that substance. Leibniz held that the world is composed of an infinity of simple substances, which he called monads. Monads are the simplest units of existence and each monad is a different simple substance which is unextended and without parts.

God is an infinitely perfect being who from an infinite number of possible worlds creates the best possible world. He cannot, however, create a perfect world, for that would be logically impossible. To do so he would have to reproduce himself exactly. God is nonextended spirit; thus, a reproduction of his qualities would be indiscernible and so nonexistent. Thus the best of all possible worlds is the one containing as much existence as possible compatible with the greatest degree of perfection. Everything in the universe unfolds according to a pre-established pattern.