The Ancient Greeks would not have understood our concept of being one thing. He's an accountant, she's a cop, he's an actor, she's an engineer. They would have told you these people, who identify with their profession, and only one profession, were not full people, but only part.
Socrates was a playwright, a mathematician, an architect, an artist, a scholar, a teacher. They never stopped learning and doing. There is so much to life, and everything is connected. The more you do, the better you get at all of it. This thought that a jack of all trades is a master of none is false. Yes, there are those who excel over everyone else by focusing on one thing, but they have missed most of what life is by focusing on one thing.