Third Man Argument

The third man argument (commonly referred to as TMA), first offered by Plato in his dialogue Parmenides, is a philosophical criticism of Plato's own theory of Forms. This argument was furthered by Aristotle who used the example of a man (hence the name of the argument) to explain this objection to Plato's theory; he posits that if a man is a man because he partakes in the form of man, then a third form would be required to explain how man and the form of man are both man, and so on, so on, ad infinitum.

[From Wikipedia: http://en.wikipedia.org/wiki/Third_man_argument]