The Republic by Plato Part 1 Page 60

Prithee, friend, do not keep your knowledge to yourself; we are a large party; and any benefit which you confer upon us will be amply rewarded. For my own part I openly declare that I am not convinced, and that I do not believe injustice to be more gainful than justice, even if uncontrolled and allowed to have free play. For, granting that there may be an unjust man who is able to commit injustice either by fraud or force, still this does not convince me of the superior advantage of injustice, and there may be others who are in the same predicament with myself. Perhaps we may be wrong; if so, you in your wisdom should convince us that we are mistaken in preferring justice to injustice.

And how am I to convince you, he said, if you are not already convinced by what I have just said; what more can I do for you?