Let’s take a look at the scene (336b):
“Now Thrasymachus had many times started out to take over the argument in the midst of our discussion, but he had been restrained by the men sitting near him, who wanted to hear the argument out. But when we paused and I said this, he could no longer keep quiet; hunched up like a wild beast, he flung himself at us as if to tear us to pieces. Then both Polemarchus and I got all in a flutter from fright. And he shouted out into our midst and said, ‘What is this nonsense that has possessed you for so long, Socrates? And why do you act like fools making way for one another? If you truly want to know what the just is, don’t only ask and gratify your love of honor by refuting whatever someone answers — you know that it is easier to ask than to answer — but answer yourself and say what you assert the just to be. And see to it you don’t tell me that it is the needful, or the helpful, or the profitable, or the gainful, or the advantageous; but tell me clearly and precisely what you mean, for I won’t accept it if you say such inanities.’ I was astounded when I heard him, and, looking at him, I was frightened. I think that if I had not seen him before he saw me, I would have been speechless. As it was, just when he began to be exasperated by the argument, I had looked at him first, so that I was able to answer him; and with just a trace of a tremor, I said: ‘Thrasymachus, don’t be hard on us. If we are making any mistake in the consideration of the arguments, Polemarchus and I, know well that we’re making an unwilling mistake. If we were searching for gold we would never willingly make way for one another in the search and ruin our chances of finding it; so don’t suppose that when we are seeking for justice, a thing more precious than a great deal of gold, we would ever foolishly give in to one another and not be as serious as we can be about bringing it to light. Don’t you suppose that, my friend! Rather, as I suppose, we are not competent. So it’s surely far more fitting for us to be pitied by you clever men than to be treated harshly.’”
Now, is Thrasymachus’s challenge fair? Should Socrates assert what he thinks justice is, rather than simply refuting everyone else? Is it really easier to ask the questions than it is to answer them?
Thrasymachus also bars the following five answers from Socrates: the needful, the helpful, the profitable, the gainful, and the advantageous; but when Socrates transforms this request, what do we get? Let’s look at 337b:
“‘Hence you knew quite well that if you asked someone how much twelve is and in asking told him beforehand, “See to it you don’t tell me, you human being, that it is two times six, or three times four, or six times two, or four times three; I won’t accept such nonsense from you” — it was plain to you, I suppose, that no one would answer a man who asks in this way.’”
How many options are listed by Socrates? 2x6, 3x4, 4x3, 6x2. Four options. Take Socrates’s question as a real question, that is, not merely rhetorical. If I asked you how much was twelve, but don’t give me one of these four options, what would you say? Both 1x12 or 12x1 presuppose that you know how much twelve is. So is there another answer?
Yes: 2x2x3.
So there is an answer to Socrates’s formulation of Thrasymachus’s question, might this imply that there is an answer to Thrasymachus’s own formulation? If justice can’t be the needful, the helpful, the profitable, the gainful, nor the advantageous, what could it be?
Have we not up until now been discussing justice as that which is fitting? That is, giving to each that which is owed, or that which they deserve, or that which is fitting to them. In fact, Socrates invokes that which is fitting at the very end of his initial response to Thrasymachus when he said, “‘…surely far more fitting for us to be pitied by you clever men than to be treated harshly.’”
Although he hints at it heavily, making reference to that which is deserved or that which is fitting four times in their exchange, Socrates does not explicitly point out the alternative; rather, he goads Thrasymachus into trying to prove that he is stronger in speech than Socrates.
Thrasymachus asks: “‘What if I could show you another answer about justice besides all these and better than they are?’” The response from everyone is that they want him to show them. Socrates is able to defer because, as he editorializes at 338ab, Thrasymachus wants to speak so he can win a good reputation.
So, what is Thrasymachus’s definition of the just?
“The just is nothing other than the advantage of the stronger.”
Now, Thrasymachus does not mean the advantage of the physically stronger, but the stronger politically, the ruling element or faction within a city. Here you can think of President Obama’s first meeting with congressional Republicans where he said matter of fact-ly: “I won.”
What this ultimately comes down to is a version of what is called legal positivism, that is, that justice is what the law says it is.
Thrasymachus gives us three regimes: tyrannical, democratic, and aristocratic. Every regime makes laws that seek to perpetuate the status quo – tyrannical regimes make laws to preserve tyranny; democratic regimes make laws to preserve democracy; aristocratic regimes make laws to preserve aristocracy. Insofar as ruling is good or advantageous, then justice is the advantage of the ruling faction, that is, the politically stronger.