“Diogenes taught the children of Xeniades, after their other lessons, to ride, and shoot, and sling, and dart. And then in the Gymnasium he did not permit the trainer to exercise them after the fashion of athletes, but exercised them himself to just the degree sufficient to give them a good colour and good health.
And the boys retained in their memory many sentences of poets and prose writers, and of Diogenes himself; and he used to give them a concise statement of everything in order to strengthen their memory.
And at home he used to teach them to wait upon themselves, contenting themselves with plain food, and drinking water.
And he accustomed them to cut their hair close, and to eschew ornament, and to go without tunics or shoes, and to keep silent, looking at nothing except themselves as they walked along.”
Source: Diogenes Laertius, Lives of the Eminent Philosophers