The Republic by Plato Part 4 Page 46

Well then, tell me, I said, whether I am right or not: You remember the original principle which we were always laying down at the foundation of the State, that one man should practise one thing only, the thing to which his nature was best adapted; — now justice is this principle or a part of it.

Yes, we often said that one man should do one thing only.

Further, we affirmed that justice was doing one’s own business, and not being a busybody; we said so again and again, and many others have said the same to us.

Yes, we said so.

Then to do one’s own business in a certain way may be assumed to be justice. Can you tell me whence I derive this inference?

I cannot, but I should like to be told.