This book is about three philosophers – Gottfried Wilhelm Leibniz, Antoine Arnauld, and Nicolas Malebranche – who happened to be together in Paris in the 1670s and their great debate over God, evil, and the meaning of life. The three were friends, at least until their intellectual and personal falling out, although they were very different personalities and came from very different national, religious, cultural and social backgrounds. Leibniz was a German Lutheran, a natural genius and polymath – with religious, scientific, mathematical, philosophical, political, and even engineering interests – and still a young man when he arrived in Paris on a diplomatic mission. Arnauld, a brilliant but very irascible theologian, was a Catholic priest. As the leading defender of the Jansenist reform movement in France, he was constantly in conflict with the ecclesiastic and political authorities; he eventually had to flee France and live the rest of his life in exile in the Netherlands. Malebranche, also a Catholic priest, was a kind and patient intellectual, somewhat slower than Arnauld both in intellect and in temper, but also the most important follower of Descartes’s philosophy in the latter half of the seventeenth-century. The debate on philosophical and theological matters between these three figures is a rich and fascinating one, giving us a glimpse into the intellectual life of early modern Europe and an understanding of why certain problems mattered so much at the time. Above all, Leibniz, Arnauld, and Malebranche disagreed on their solutions to the so-called “problem of evil.” Why is there sin, suffering, and other evils in a world supposedly created by a wise, just, and all-powerful God? Why are there droughts, floods, and other natural disasters that bring great suffering among many innocents? Why are there wars, murders, and other miseries brought about by human wickedness? Why do vicious people flourish while bad things happen to good people? A philosophical solution to this problem is called a “theodicy”, or explanation of God’s justice. These three thinkers disagreed not only on how best to account for divine justice but also, correspondingly, on the proper way to conceive of God and His manner of acting. Is God a rational being like us, a kind of “person” who always acts for the sake of some good and according to objective values? Or is God ultimately an arbitrary being on whose incomprehensible will alone everything depends and thus who is not bound by any objective values independent of what He wills? And what does this mean for the intelligibility of the universe itself? Is God’s creation the product of a rational activity guided by what is good, or is it, at its most fundamental level, an arbitrary product governed by divine fiat? In short, is this the best of all possible worlds?