What does Aristotle argue about Plato's theory of forms?

Aristotle is critical (in his work Metaphysics) of Plato’s theory as espoused in the latter’s main works because the element of participation in this theory is, according to Aristotle, unclear (in what sense can individuals be said to participate in the Forms (or Ideas)?). The Forms’ independent existence is also eluding: suppose that there are no individuals that exhibit a quality, is this quality then still to be supposed to exist, and, if so, how?

Aristotle points to some incomprehensible consequences of Plato’s theory, such as the ‘one over many’ argument – that there will be Forms of negations –, and the argument that thought has an object: when an object has ceased to be, there will be Forms of things that can pass (since people have images of them). The ‘third man’ argument is perhaps the most famous one. Simply put, it consists in an infinite regression that follows from the positing of Forms: suppose that a Form is to account for individuals (e.g., the Form of ‘Man’ for individual men or people). If the quality of man-ness that the individuals and the Form share is to be explained, another Form must be introduced, and so on.

In order to escape these problems, Aristotle takes a position that considers Forms in a more ‘modest’ guise, Forms constituting the nature (essence) of that which a thing is. They are not something distinct from the individual, but constitute it. For example, a peregrine falcon’s Form is actually being a peregrine falcon, which means, inter alia, (to have the ability) to take care of its young, to fly at high speeds and – in the process – to catch preys.

Interestingly, Plato is critical of his own theory in his dialogue Parmenides, in which the philosopher Parmenides is an important interlocutor (though only literarily, not historically). The ‘third man’ argument, for example, is discussed in 132a, b, where Plato uses ‘great’ as an example: if something is great and the Form of ‘Great’ is to account for the greatness, once one acknowledges the fact that the great things and the Form of ‘Great’ share the quality of greatness, another Form must be introduced to explain this, and then the same situation presents itself, the number of Forms being infinite. (Plato can be said to be a little clearer here than Aristotle here, perhaps due to the form (no pun intended), viz. that of the dialogue, that Plato has chosen.)

Aristotle Theory Of Forms