with another, or the change of one into another, is the greatest harm to the State, and may be most justly termed evil-doing?

Precisely.

And the greatest degree of evil-doing to one’s own city would be termed by you injustice?

Certainly.

This then is injustice; and on the other hand when the trader, the auxiliary, and the guardian each do their own business, that is justice, and will make the city just.

I agree with you.

We will not, I said, be over-positive as yet; but if, on trial, this conception of justice be verified in the individual as well as in the State, there will be no longer any room for doubt; if it be not verified, we must have a fresh enquiry.