Plato's Arguments for Forms

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...