Cicero On the Ends of Good and Evil
"For if the whole of a man's life is to be measured by the standard of pleasure, you see how much trouble is in store for us. We must consider not what is pleasant, but what is consistent with the dignity of human nature; for there is a certain dignity in man, which the beasts do not share. If we lose sight of this, we shall be unable to maintain any standard of conduct." (Cicero, De Finibus 2.44).
