"The past is certain, the future obscure."
"I know that I know nothing."
"The measure of a man is what he does with power."
"Do not say a little in many words but a great deal in a few."
"What has been affirmed without proof can also be denied without proof."
"πάντα ῥεῖ (panta rhei) - everything flows"
"The difficulty is not so great to die for a friend, as to find a friend worth dying for."
"Nothing exists except atoms and empty space; everything else is opinion."
"The whole is more than the sum of its parts."