Perfectionism is one of the great moralities of the Western tradition. It holds that certain states of humans, such as knowledge, achievement, and friendship, are good apart from any pleasure they may bring, and that the morally right act is always the one that most promotes these states. Defined more narrowly, perfectionism identifies the human good by reference to human nature: if knowledge and achievement are good, it is because they realize aspects of human nature. This book gives an account of perfectionism, first in the narrower sense, analysing its central concepts and defending a theory of human nature in which rationality plays a central role. It then uses this theory to construct an elaborate account of the intrinsic value of beliefs and actions that embody rationality, and applies this account to political questions about liberty and equality. The book attempts to formulate the most defensible version of perfectionism, using contemporary analytic techniques. It aims both to regain for perfectionism a central place in contemporary moral debate and to shed light on the writings of classical perfectionists such as Aristotle, Aquinas, Hegel, Marx, Nietzsche, and T.H. Green.