This book offers a sympathetic explanation of the origin of the Theory of Forms that is true both to the dialogues and to Plato's place in history. The author's explanation makes the development of Plato's thought part of an intellectual and philosophical history that begins in the pre-Socratic period, extends through Socrates and the Sophists, and continues into the twentieth century. The explanation provides a unified reading of three passages that scholars have long recognized as keys to Plato's thought about the Forms, but which have proved stubbornly resistant to interpretation, both individually and as a group: the intellectual autobiography in the Phaedo; the discussion of the philosopher and the lover of spectacles in the Republic; and, the discussion of starting points, the Receptacle, and the four kinds of stuff in the Timaeus. The book will be of special interest to scholars of ancient philosophy, it will also be of interest to philosophers in general, graduate students, and advanced undergraduate students.