This collection of essays--some of them already classics and some of them published here for the first time--will become the indispensible sourcebook for students of evolutionary epistemology, which has been called the most important development in the theory of knowledge since the eighteenth century. Together, these chapters give a systematic exposition of growth of knowledge by blind variation and selective retention, the nonjustificational rationality which underlies evolutionary epistemology, and the application of its principles to the sociology of knowledge.