Plato thought that in addition to the changeable, extended bodies we perceive around us, there are also unchangeable, extensionless entities, not perceptible by the senses, that structure the world and our knowledge of it. He called such an entity a ‘Form’ (eidos) or ‘Idea’ (idea), or referred to it by such phrases as ‘the such-and-such itself’. Thus in addition to individual beautiful people and things, there is also the Form of Beauty, or the Beautiful Itself.
It may be speculated that Plato’s Presocratic predecessors gave some impetus to this theory. It is a certainty that Socrates was the major influence on it, through his search for the definitions of ethical terms. The features that a definition must have in order to satisfy Socrates’ criteria of adequacy foreshadow the features that Forms have in Plato’s theory.
Beginning with his Meno, Plato turned his attention to the presuppositions of Socrates’ investigation, and the preconditions of its possibility: what has to be true about virtue, knowledge and our souls if Socratic cross-examination is to have any hope of success? He answers these questions with a set of doctrines – the existence of Forms, the soul’s immortality and its knowledge of Forms through recollection – which are then developed and displayed in the great dialogues of his middle period, the Phaedo, Symposium, Phaedrus and Republic. Not all of Plato’s thoughts on Forms are on display in the middle-period theory, but this is the theory of Forms that has been far and away the most influential historically, and the one that is most commonly intended when people refer to ‘Plato’s Forms’.
The dialogues of Plato’s later period present a number of puzzles. That his views developed will be agreed by all: in the Sophist, Statesman and Philebus Plato is clearly pushing his metaphysical investigations in new directions. What is less clear is the degree of continuity or rupture between old and new – the Parmenides has sometimes been taken to signal Plato’s wholesale rejection of the middle-period theory, whereas the Timaeus seems to confirm his endorsement of it. Further complicating matters, Aristotle reports that Plato in his last period based the Forms somehow on numbers. The reported material is obscure in itself and also hard to integrate with any of the material from Plato’s dialogues.
Much of our current understanding of Plato’s middle-period theory comes from a group of arguments that advert to differences between Forms and sensible objects or properties. These arguments tend to support Aristotle’s report that the theory arose from a collision between Socrates’ views on definition and Heraclitean views on flux. The general form of the argument claims that definitions, or knowledge, require the existence of a class of entities with certain features, and that sensibles lack those features. It concludes that there exists a class of entities distinct from the familiar sensibles, namely the Forms.
But as often in historical studies, the arguments themselves are silent or ambiguous on many of the points that critics most wish to determine: whether Plato thought Forms exist separately from particulars, whether he treated them as Aristotelian substances, whether it is possible to have knowledge of sensible objects, whether Plato came to reject the middle-period theory, and so on. For the second half of the twentieth century, the tendency was for interpreters to settle the remaining interpretative issues by ascribing to Plato their own philosophical preferences, justifying this by appeal to ‘interpretative charity’.
The practice of basing interpretations of Plato’s Forms solely on a handful of arguments was a mistake; the increasing tendency to broaden the evidentiary base is a salutary development. Where the interpretation of an argument has left a question unresolved, the consideration of Plato’s myths and metaphors may sometimes lend strong weight to one side or the other. An example: Plato’s depictions of particulars make it highly implausible that the ‘imperfection’ in particulars to which some arguments advert is merely the compresence of opposites.
Most of Plato’s successors in the early Academy kept up the Forms. Aristotle’s writing are full of references to them, and they left visible imprints on his own theory. The Hellenistic period witnessed a blanket rejection of all immaterial entities, but even here the influence of the Forms can still be discerned around the edges. The revival of Platonism at the end of the Hellenistic period saw Forms returned to philosophical respectability.
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