Now listen to what I said would be the first topic—the nature and origin of justice. By nature, they say, to commit injustice is a good and to suffer it is an evil, but that the excess of evil in being wronged is greater than the excess of good in doing wrong. So that when men do wrong and are wronged by one another and taste of both, those who lack the power to avoid the one and take the other determine that it is for their profit to make a compact with one another neither to commit nor to suffer injustice; and that this is the beginning of legislation and covenants between men, and that they name the commandment of the law the lawful and the just, and that this is the genesis and essential nature of justice—a compromise between the best, which is to do wrong with impunity, and the worst, which is to be wronged and be impotent to get one's revenge. Justice, they tell us, being mid-way between the two, is accepted and approved, not as a real good, but as a thing honored in the lack of vigor to do injustice, since anyone who had the power to do it and was in reality 'a man' would never make a compact with anybody either to wrong nor to be wronged; for he would be mad. The nature, then, of justice is this and such as this, Socrates, and such are the conditions in which it originates, according to the theory. Glaucon in Plato's Republic, 358E-359B, translated by Paul Shorey.
In his On Politics, Alan Ryan calls this the "commonsensical" view about the origin of justice and, quite naturally, sees in it an anticipation of the social contract theory of Leviathan. In fact, Ryan is puzzled why it falls flat in the Republic. Unfortunately, Ryan does not explain why he is puzzled, except, perhaps, to note that the Greeks were familiar with contracts and that Glaucon, who is offering a composite view of what folk like Thrasymachus would say, is subtle enough ("philosophically deft") to note that the contract is not an actual contract, but one that would have been made. In what follows I'll call this the sophistic social contract.
The sophistic social contract is grounded in a view of human nature: [A] that to suffer the harms of injustice outweighs the benefits of doing injustice ("the excess of evil in being wronged is greater than the excess of good in doing wrong.") Let's stipulate for the sake of argument that [A] is true psychologically, although one wonders if the person doing harm would agree (I return below). [A] seems to presuppose [A*] a kind of hedonic calculus. That such a hedonic calculus is presupposed is not itself a conceptual problem, although one wonders if one is fully entitled to it in a pre-social contract state. For, as Hobbes notes in chapter 16 of the the Leviathan, there private appetites are the measure of goodness/badness. Part of the point of the social contract is, in fact, to supply common measures.
This last point connects to the more serious problem with [A] and that is that it seems to presuppose (justice) what it is supposed to explain (origin of justice). I'll allow that one can save the sophistic contract by allowing that injustice by nature is conceptually not identical to the injustice grounded by social contract. The distinction is clearly in Glaucon's presentation of it, but the story that would help explain how such a distinction is supposed to work is absent.
The second notable feature of the sophistic social contract is that [B] it is grounded in the experience of those that cannot avoid being harmed by a state of nature. This entails that [C] not everone is party to the sophistic state of nature, for the naturally very strong or charismatic are not evidently included. And one can well wonder why, if the very strong and charismatic are not included, the sophistic social contract would get off the ground or be enduringly stable One can see in Hobbes's version of the very bad state of nature (or (recall) Master Mo's or Suarez's (recall)) as a way of addressing this obvious gap in the argument--it's then very hard to avoid the negative effects of lack of justice. This, in turn, is grounded in natural equality (recall).
These key assumptions of the sophisgtic social contract [B-C] rely on the underlying assumption that the social contract is grounded in the experiences of ordinary human beings not the exceptional types. That is, [D] in the sophistic social contract not everyone is equal. Rather, (and this anticipates Nietzsche's criticism of the social contract) it is a way for the many relatively weak to contain the few relatively strong, who have no initial incentive to join into this socialc ontract.* For them it is not evidently profitable--for them it would be madness to join. This is why [E] the sophistic social contract makes no mention of unanimity (which is required in Hobbes).
So, lurking behind the sophistic social contract is the idea that [F] justice is a way of the many weak imposing their collective strength on the strong and calling it a "compromise." That is to say, in the sophistic social contract, [G] the origin of justice is simultaneously the origin of ideology. This is barely disguised in this account (it's a feature not a bug). This [G] is not itself an objection to the theory. But [F] is problematic because it [F*] presupposes that the weak solve a rather tricky coordination problem and so, in part, assumes what it is supposed to explain.
To sum up. Recall that the Hobbesian social contract assumes this: first, one assumes that (i) in the state of nature it is a contract among (ii) free and equal (iii) individuals who (iv) transfer (some of their) rights to a collective and/or some sovereign and in (v) the process are transformed from a multitude to a unity. The key point is (vi) consent is required for submission or obedience to legitimate authority. The sophistic social contract rejects (ii) and so really has no right to (v), which goes unmentioned. And (vi) does not apply to all because the strong would never consent to the social contract.
The preceding makes it sound that I have a low opinion of the sophistic social contract. But, of course, its lack of reliance on the methodological assumption of natural equality makes it, in some sense, more plausible/realistic. And by grounding it in the good consequences (profit) that are purportedly to follow from it, it does not presuppose a distinctly moral sensitivity in the contracting parties (which hinders many later accounts of the social contract). The problem is that it then also reveals that the sophistic social contract is singularly bad at explaining or motivating continued compliance from those who are most dangerous.+ And so while it is very natural for many reasons to read Hobbes as the heir to the sophists, Glaucon's representation (which should not be the last word, of course) suggests that there are also very important differences.