As he didn't write his thinking, nor we have found a written prove of his ideals, we can tell that he was greatly brave, to think different in such a time... You might not know him, but he knew all of us before we even borned, here are some of his quotes:
“The only true wisdom is in knowing you know nothing.” ... “The unexamined life is not worth living.” ... “I cannot teach anybody anything. ... “There is only one good, knowledge, and one evil, ignorance.” ... “Be kind, for everyone you meet is fighting a hard battle.” ... “Wonder is the beginning of wisdom.”