In writing this book, I had in mind a number of audiences, and not just academic ones. I hope to reach people interested in European intellectual history and in early modern religious thought, as well as those who value philosophical and theological problems about God, the world, and humanity. But the book is also about the relationship between morality and religion. Can God’s will serve as the source of, or even as a guide to what is moral? Or is morality, good and bad and right and wrong, something completely independent of any religious or theological doctrines? Does God will something because it is independently good, or is it good for the sole reason that God wills it? This is a millennia-old debate, going back to Plato and his dialogue Euthyphro – in which Socrates argues against the view that something is pious just because the gods love it – but it continued to inform philosophical thinking throughout the Middle Ages and the modern period. The Bible says that God looked at what He had created and saw that it was good. But did God create it because it was good, or was it good simply because God created it? The problem is no less relevant today, particularly as so many people believe that ethical principles and ethical behavior must be grounded in religious beliefs, and that without a God there can be no morality, which I regard as a dangerous conflation between two domains. This book arises out of my ongoing research into seventeenth-century thought and especially the intersection of philosophy, science, and religion in the period and its relevance for our lives today – particularly since much of what we think of as “modern” first arises in the seventeenth century. Studying ideas in the seventeenth century is most definitely not of solely antiquarian or historical interest, for we are very much products of the Enlightenment, both its moderate version (Descartes, Locke, Voltaire) and its radical strain (Spinoza).