No artist or sage or ruler errs at the time when he is what his name implies; though he is commonly said to err, and I adopted the common mode of speaking.

But to be perfectly accurate, since you are such a lover of accuracy, we should say that the ruler, in so far as he is the ruler, is unerring, and, being unerring, always commands that which is for his own interest; and the subject is required to execute his commands; and therefore, as I said at first and now repeat, justice is the interest of the stronger.
Indeed, Thrasymachus, and do I really appear to you to argue like an informer?
Certainly, he replied.
And you suppose that I ask these questions with any design of injuring you in the argument?
Nay, he replied, 'suppose' is not the word--I know it; but you will be found out, and by sheer force of argument you will never prevail.
I shall not make the attempt, my dear man; but to avoid any misunderstanding occurring between us in future, let me ask, in what sense do you speak of a ruler or stronger whose interest, as you were saying, he being the superior, it is just that the inferior should execute--is he a ruler in the popular or in the strict sense of the term?
In the strictest of all senses, he said.

And now cheat and play the informer if you can; I ask no quarter at your hands.

But you never will be able, never.
And do you imagine, I said, that I am such a madman as to try and cheat, Thrasymachus?
I might as well shave a lion.
Why, he said, you made the attempt a minute ago, and you failed.
Enough, I said, of these civilities.

It will be better that I should ask you a question: Is the physician, taken in that strict sense of which you are speaking, a healer of the sick or a maker of money?
And remember that I am now speaking of the true physician.
A healer of the sick, he replied.
And the pilot--that is to say, the true pilot--is he a captain of sailors or a mere sailor?
A captain of sailors.
The circumstance that he sails in the ship is not to be taken into account; neither is he to be called a sailor; the name pilot by which he is distinguished has nothing to do with sailing, but is significant of his skill and of his authority over the sailors.
Very true, he said.
Now, I said, every art has an interest?
Certainly.
For which the art has to consider and provide?
Yes, that is the aim of art.
And the interest of any art is the perfection of it--this and nothing else?
What do you mean?
I mean what I may illustrate negatively by the example of the body.
Suppose you were to ask me whether the body is self-sufficing or has wants, I should reply: Certainly the body has wants; for the body may be ill and require to be cured, and has therefore interests to which the art of medicine ministers; and this is the origin and intention of medicine, as you will acknowledge.