Full Idea
Socrates and Plato are right: whatever man does, he always acts for the good; that is, in a way that seems to him good (useful) according to the degree of his intellect, the prevailing measure of his rationality.
Gist of Idea
People always do what they think is right, according to the degree of their intellect
Source
Friedrich Nietzsche (Human, All Too Human [1878], 102)
Book Reference
Nietzsche,Friedrich: 'Human, All Too Human', ed/tr. Faber,Marion [Penguin 1994], p.71
A Reaction
I associate this doctrine much more with Socrates than with Plato - but Nietzsche was a great classical scholar.