Plato emphatically believed in forms, while his nephew Speusippus did not. Because of this conflict, Plato and Speusippus must have had many debates concerning the existence of the forms, yet no one in antiquity ever mentioned them. Such forgotten debates is the topic of this volume. These debates include, besides those between Plato and Speusippus, those between the Eleatics and the sophists and those that Plato had with other philosophers when he first developed his theory of forms. Can we find any traces of such debates? Can we outline their development and determine their outcome? Does the existence of these debates change our account of the history of ancient Greek philosophy? The answer to these questions is a surprising yes, and the details can be found in this unique study. By bringing these forgotten debates to the fore, this volume reveals how obscure philosophers such as Speusippus and Cratylus were more important than anyone has imagined, while Parmenides, often thought of as a philosophical giant, correspondingly shrinks in influence.