Should you be willing to read [Xenophon's Anabasis] very carefully you shall discover how ... to deceive one’s enemies to their harm and one’s friend… - Dio Chrysostom

Just as the good physician should go and offer his services where the sick are most numerous, so, said [Diogenes], the man of wisdom should take up his abode where fools are thickest in order to convict them of their folly and reprove them.

When some people urged that it is impossible for man to live like the animals owing to the tenderness of his flesh and because he is naked and unprotected, [Diogenes] would say in reply that men are so very tender because of their mode of life. ... Man’s ingenuity and his discovering and contriving so many helps to life had not been altogether advantageous to later generations, since men do not employ their cleverness to promote courage or justice, but to procure pleasure.

If in the guise of philosophers, they [declaim speeches] with a view to their own profit and reputation, and not to improve you, that indeed is shocking. For it is as if a physician when visiting patients should disregard their treatment and their restoration to health, and should bring them flowers and courtesans and perfume.