The Republic - pg. 123

"And in truth justice was, as it seems, something of this sort;
however, not with respect to a man's minding his external
business, but with respect to what is within, with respect to
what truly concerns him and his own. He doesn't let each
part in him mind other people's business or the three classes
in the soul meddle with each other, but really sets his own house
in good order and rules himself; he arranges himself, because
his own friend, and harmonizes the three parts, exactly like
three notes in a harmonic scale, lowest, highest and middle.
And if there are some other parts in between, he binds them
together and becomes entirely one from many, moderate and
harmonized. Then, and only then, he acts, if he does act in some
way - either concerning the acquisition of money, or the care of
body, or something political, or concerning private contracts.
In all these actions he believes and names a just and fine action
one that preserves and helps to produce this condition, and
wisdom the knowledge that supervises this action; while he
believes and names an unjust action one that undoes this condition,
and lack of learning, in its turn, the opinion that supervises
this action."