In making this claim Rawls rules out the suggestion that legal obligation could be grounded on a specific principle of its own, not least because, he states, no one wishes to argue this. This point is made rather swiftly and seems to indicate that Rawls could conceive of no form of jurisprudence that attempted to find legal obligation to be of independent standing.
The next point that is raised is the sense of "obligation" that is implied in claiming that there is a moral obligation to obey the law and Rawls claims that the sense of obligation in question is one connected to institutional rules but this apparently vacuous claim is grounded on a specific sense of an "institution" as something that is related to what he termed an "association" in the previous paper on the sense of justice. It is characterised, in other words, by "voluntary acts" in which obligations arise from others. This does imply the slightly odd view that legal obligations have been entered into voluntarily by us and indicates a kind of precedence of moral obligation in relation to law, an impression that is furthered by Rawls stating that crimes of violence are wrong "independently of there being a legal system", something surely true but which shows that law requires assent of a kind to moral principles to gain a footing in the first place. (The later debate with Habermas will partly concern this question.)
In looking further at the special questions arising from thinking about the law Rawls fills in details of a legal system that fits the bill of having a clear and, in a wide sense of the term, "voluntary", hold on us. The first point is that such a system has to satisfy the rule of law meaning that under its reign there are clear and public criteria and that "similar cases are treated similarly". The feature of the rule of law is central to the form of law rather than to specific legal contents. But Rawls also supposes that the legal order that claims such "voluntary" consent also belongs to a democratic system in the sense of ensuring equal citizenship, basic freedoms and general equality of the suffrage.
The discussion subsequently moves on to the question of how the moral obligation to obey the law works given that it appears to imply that we have obligations to obey laws we might think are unjust and even that obeying them will not produce good "overall". One of the points Rawls makes in looking at these apparent problems is that voting, as a basis for law-making, has no characteristics in itself that ensures the best decisions (or even good ones) will be made. It simply decides which policy will be put into effect. So constitutions create mechanisms by which conflicting views are brought to a resolution, one which need not at all settle the dispute in question. The invocation of the duty of "fair play" concerns precisely the basis for following the law in such a situation as we have an obligation not towards legislators but rather towards other citizens and it is the other citizens who have a basis for indignation if we do not follow the law. But this does require that the general constitutional procedures are themselves just.
It is to specify the basis for viewing such a constitution as just that Rawls subsequently proceeds and these bases turn out, perhaps unsurprisingly, to include adoption of his principles of equal liberty and the acceptance that inequality is arbitrary unless it can be shown to be to everyone's advantage. Under such conditions even a law that is unjust still mandates an obligation that it be followed. Noticeably, Rawls' conditions here are stricter than those Kant appears to adopt when discussing the problems with any claimed "right" to revolution in the Doctrine of Right. Kant's account is part of a general argument concerning the nature of right but it does not require anything like the second principle Rawls adopts.
Rawls next looks at the problem with viewing laws from the standpoint of net overall good rather than in terms of a general moral obligation to obey the law. The basic point of this discussion is to bring out problems with basing a system of justice on the principle of utility. Rawls states that unjust institutions cannot be identified by the principle of utility and that the obligation to obey the law cannot be overridden by this principle. However, Rawls confesses that to establish this claim would require "a deeper argument based on an analysis of the concept of right" and he does not give such an argument, a sense in which the argument of the essay does not reach the level of Kant's Doctrine of Right. Rather than provide such an account of the concept of right Rawls reverts back to the general point that to attribute to justice an absolute weight is to interpret the concept of right by means of the sense of justice as grounded in an initial position of equality (thus invoking the original position).
The general question of the obligation to obey the law is settled for Rawls by a series of tests including those that would look at the justice of the constitution, the depth of the injustice the law enacted, whether those enacting it wish the injustice to result and whether we can realistically hope to repeal the law. This already looks towards questions concerned with civil disobedience but these questions aren't here taken further.
The legal system generally is looked at in this essay as a system of social cooperation that requires equilibrium between conflicting claims. The essay seems thus to be something of an intermediate stage in Rawls' thinking as we need more detail yet of both the general conception of the social/legal system and of the means by which civil disobedience is to be understood.