Why did Plato define a human as a featherless biped?

According to Diogenes Laërtius' third-century Lives and Opinions of the Eminent Philosophers, Plato was applauded for his definition of man as a featherless biped, so Diogenes the Cynic “plucked the feathers from a cock, brought it to Plato's school, and said, 'Here is Plato's man. read more

Plato was smart - he probably knew that “featherless biped” did not actually say very much about what a human is. But for the ancient Greeks, who did not know much about Kangaroos but did know about feathered bipeds like birds, this definition worked amazingly well to basically include all humans and exclude all else. read more

Pumpkins, no; human being, yes; human as featherless biped, maybe. In fact “featherless biped” conjures up an enterprise of taxonomy. First you sort animals by the number of feet they have, which means you have a category for spiders with eight, flies with six, horses with four, and so on. read more

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. read more

Plato’s dialogues offer glimpses of how philosophy would have been practiced at his school, as a collective and cooperative quest for the demarcation of “species” or “kinds” of things. Moral terms, political concepts and biological entities were investigated and defined. Pumpkins, no; human being, yes; human as featherless biped, maybe. read more

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. read more