Once upon a time, philosophy was a dangerous business-and for no one more so than for Baruch Spinoza, the seventeenth-century philosopher vilified by theologians and political authorities everywhere as "the atheist Jew." As his inflammatory manuscripts circulated underground, Spinoza lived a humble existence in The Hague, grinding optical lenses to make ends meet. Meanwhile, in the glittering salons of Paris, Gottfried Wilhelm Leibniz was climbing...