In order to be truly free, must you act arbitrarily? If an event did not happen, could it have happened? Since there is evil, and God could have made the world without evil, did God fail to pick the best course? Grappling with such simple—yet still intriguing—puzzles, Leibniz was able to present attractively his new theories of the real and the phenomenal, freewill and determinism, and the relation between minds and bodies.

Theodicy was Leibniz's only book-length work to be published in his lifetime, and for many years the work by which he was known to the world. Fully at home with the latest scientific advances, Leibniz ultimately rejected the new atomistic philosophies of Descartes, Gassendi, and Hobbes, and drew upon the old cosmology of Aristotelian scholasticism. There could be no conflict, he argued, between faith and reason, freedom and necessity, natural and divine law. Ingeniously defending his postulate of pre-established harmony, Leibniz made important advances in the precise analysis of concepts.