If we are to understand why Plato had a theory of Forms, we must explain, firstly, why he thought it necessary to depart from the ontology of the Socratic dialogues; secondly, why he then posited the existence of entities that have the characteristics that he ascribes to Forms (entities that are 'unmixed', 'unchanging', 'in every way being' and so on); and thirdly, why Plato took this course when other philosophers have not done so (and even he himself and his immediate pupils were later to modify or abandon the theory). In this study, Robert William Jordan discovers an answer to these questions where we might expect to find one - namely in the arguments Plato gives us in favour of the hypothesis that there are Forms. These arguments, on analysis, reveal not just a concern with the nature of knowledge and explanation, but an interest in the analysis of the apparent contradictions that Plato in his middle period thought to be presented to the intellect by the sensible world. These contradictions, he then thought, could not be resolved except by those with knowledge of the Forms.