In this seminar we will explore the thought of early modern rationalists, including Descartes, Spinoza, Lady Conway, and Leibniz. The rationalists pursued certain necessary truths derived from self-evident principles and innate ideas. Although sometimes distrustful of empirical investigations, they still contributed to scientific advancement, especially in physics and mathematics. Some topics we will discuss include Descartes' substance dualism, and the connected problem of mind-body interaction, the rationalists' views on free will, Spinoza's necessitarianism, and Leibniz's famous pronouncement that we live in the best possible world.