'Know thyself' is a good saying, but not in all situations. In many it is better to say 'know others.'

He who knows others is wise; he who knows himself is enlightened.

The ancient precept, “Know thyself,” and the modern precept, “Study nature,” become at last one maxim.

One has to know the size of one's stomach.

You need to know things the others don't know. It's what no one knows about you that allows you to know yourself.

No one is free who has not obtained the empire of himself. No man is free who cannot command himself.