Our inner thoughts slowly start to match how we act on the outside. When Socrates got angry, he would lower his voice and speak very little. This showed he was trying to control himself. His friends learned to spot this behavior and would call him out for being angry. He didn't mind being accused of hiding his anger. How could he not be pleased that many people could see his anger, but none actually suffered from it? They would have suffered, though, if he hadn't given his friends the same right to criticize him that he used with them.
Our inward thoughts gradually become influenced by our outward demeanour. With Socrates it was a sign of anger when he lowered his voice, and became sparing of speech; it was evident at such times that he was exercising restraint over himself. His friends, consequently, used to detect him acting thus, and convict him of being angry; nor was he displeased at being charged with concealment of anger; indeed, how could he help being glad that many men should perceive his anger, yet that none should feel it? they would however, have felt it had not he granted to his friends the same right of criticizing his own conduct which he himself assumed over theirs.