This concise survey, accessible to students and general readers alike, traces the main elements of rationalism from the classical period to the present day. It contains a lucid account of the arguments of the great seventeenth-century rationalists, Descartes, Spinoza, and Leibniz, on scientific knowledge, mind and body, and freedom and necessity, and compares these with the empiricist counter-arguments of Locke and Hume, culminating in the great synthesis of Kant. Later sections discuss the ideas of Hegel, Russell, Wittgenstein, Ayer, Quine, Kripke, Chomsky, and Popper, along with rationalist and anti-rationalist elements in modern ethics.