Diogenes Walked into Plato's Academy with a Plucked Chicken

In Book 6, Chapter 40 of Lives of Eminent Philosophers, Diogenes Laertius tells a short story about a chicken. The Academy had been pleased with one of Plato's definitions: man is a featherless biped — two legs, no feathers, neat. So Diogenes the Cynic, who lived in a clay jar and made a career out of embarrassing the well-dressed, went and got a rooster. He plucked it. He carried it into the lecture hall. He set it down and said, in effect: here is Plato's man.

The Academy, according to Laertius, amended the definition. Man is a featherless biped with broad, flat nails.

The joke is doing more work than it looks. Diogenes isn't quibbling about poultry; he's pointing at how a definition by negation — not this, not that — can wrap itself around any number of things you didn't mean. Strip a chicken and the formula catches it. Add a clause to exclude the chicken and you've admitted the formula was incomplete. Keep adding clauses and you have a list of features, not a definition. Plato's school, in the anecdote, takes the bait and patches.

The historical part of all this is hazier than the philosophical part. Diogenes Laertius was writing in the third century CE, roughly six hundred years after Diogenes of Sinope died, and his Lives is a magpie's nest of anecdotes whose sourcing he rarely shares. The plucked-chicken story may be folklore that found a good home in his book rather than something that happened in any particular afternoon at the Academy.

What survives is the move. A definition is supposed to carve nature at the joints; if a Cynic with a dead bird can wreck it in one sentence, the joints were in the wrong place.