Not many seminal works of philosophy are the product of an unoccupied mind in the midst of a failed mining project, but that, it seems, was the story behind Leibniz’s Discourse on Metaphysics. At the beginning of January 1686, he travelled to the Harz mountains to continue his (ultimately ill-fated) project to improve the productivity of the mines there via wind machines and water pumps of his own invention. When the work stalled for a few days, Leibniz did what he did best: he thought. At the beginning of February 1686, he remarked to a friend that as he had nothing to do for a few days, he had “written a short discourse on metaphysics”, a remark from which the text subsequently derived its name since Leibniz left it untitled. That “short discourse”—never published in his lifetime—would later become one of his most important philosophical works, as it captures the moment when Leibniz’s philosophical system came together.

He identified the main topics of the Discourse as grace, the concurrence of God, miracles, the cause of sin, the origin of evil, the soul’s immortality, and ideas. This is rather misleading, however, since these topics occupy less than a third of the text. A more accurate thematic structure of the Discourse’s 37 sections would be: God and his choice of the best (§§1–7), substance (§§8–16), physics (§§17–22), and the relationship between God and minds (§§23–37). What follows is a primer on the main topics and lines of argument in the text.

Starting with God

Leibniz does not begin with doubt, or with the external world, or even with the self, but with God, defined as “an absolutely perfect being” who possesses the supreme degree of power and wisdom. From this, Leibniz claims, it follows that God always acts in the most perfect way, as no other way of acting is consistent with the nature of God. Consider the alternative position defended by Malebranche, that God could have made a more perfect world but chose not to do so. Leibniz rejects this outright: to act with less perfection than one is capable of, he argues, is to act imperfectly, and that cannot be attributed to a perfect being.

From this, Leibniz claims, it follows that God always acts in the most perfect way… 

Since God always acts in the most perfect way, Leibniz infers that the world we inhabit is not merely good, but rather the best possible world. At first glance, the idea can seem hard to accept, not least because the world contains suffering, injustice, and countless imperfections. Leibniz is well aware of this. But for him, the best …