Plato's definition of "Man" is subject to interpretation, but one famous anecdote gives an explicit (if satirical) Platonic definition. Based on the Dialogues, animals were classified into quadrupeds and bipeds, and then divided among those with feathers (birds) and those without. Plato (and possibly Socrates) commented that Man stood upright on two legs without feathers, making him distinct from other animals; this definition was rebuffed by Diogenes of Sinope, who brought Plato a plucked chicken as an example of "Plato's Man." After consideration, Plato amended the definition to include "broad, flat nails," differentiating from a plucked chicken's talons. His definition, then, is that "Man is an upright, featherless biped with broad, flat nails."

This, of course, leaves out all the other ways in which Humankind differentiates itself from animals. In The Republic, Plato postulates that Man differentiates from animals in three ways: the soul, which is immortal; the desire for and acquisition of knowledge; and the tendency of Man to become social and political. Each of these sets Man apart from apes, for example, which only change their instincts and habits when forced by outside means.