Discourse on Metaphysics

A short treatise in thirty-seven articles, written in 1686 and the first full sketch of Leibniz's philosophy. He sent summaries of its articles to Antoine Arnauld, and their correspondence followed, but the text itself was not published until the nineteenth century. It opens with a God who acts in the most perfect way, against the view that things are good merely because God wills them. Its central idea is the complete concept of an individual substance, which contains everything that will ever be true of it. Leibniz explains why this does not make everything necessary: Caesar's crossing of the Rubicon is contained in his concept, yet its opposite involves no contradiction, so it is certain but contingent. Each substance is like a world apart, expressing the whole universe. The Discourse also criticises Descartes's measure of the quantity of motion, defends final causes in physics, and closes with minds as citizens of a city ruled by God.

It is the best short way into Leibniz's thought as he first worked it out, and its problem of how to square a complete concept with genuine contingency remains a hard puzzle about freedom and necessity.