The two principles of justice described in Chapter II of Theory are not set in their full context until the "original position" has been described. For Kantians this can be described by saying the initial description of the two principles of justice in Chapter II is akin to a "metaphysical deduction" whilst the introduction of the "original position" provides the "transcendental deduction" of the right to use the 2 principles. However, for the "original position" to act in this justificatory way it first has to be defended as the appropriate way of conceiving the "initial situation", a better one in its way than classic pictures of the "state of nature". Once this point is made it is evident that the defence of the "original position" does have an important role in Rawls' political philosophy though it is, in fact, treated very briefly.
The first two sections of Chapter III are the place where this "defence" of the "original position" as the appropriate way of conceiving the "initial situation" is provided with its basic justification. Rawls presents the "initial situation" as a simplified choice problem by analogy with the use of equilibrium in economic theories and it includes conditions which it is thought reasonable to impose on the choice of principles. It also has to be described in a way that participants in the choice situation would agree was fair and Rawls views this as requiring the removal of all contingencies so that the right way to construe the "initial situation" is in terms of pure procedural justice.
The capturing of the "initial situation" as an "original position" is one that involves accounting for our moral judgments including our possession of a sense of justice. As Rawls writes: "Justice as fairness is a theory of our moral sentiments as manifested by our considered judgments in reflective equilibrium". So the "original position" mirrors this in providing a pure procedural situation that allows these "considered judgments" to be set into a "reflective equilibrium". The reference to reflective equilibrium is meant as a way of indicating the difference between social theory and economic theory as the place of equilibrium in the latter is as an automatic mode of operation whereas in the former the arrival at a situation of equilibrium is rather taken to be a consequence of a rightly constructed choice situation.
The reasoning employed in the construction of this situation is, as Rawls admits, "highly intuitive throughout" and so the account of this construction does not meet the task of defeating intuitionist scepticism despite the presumption we might have got from Chapter 1's treatment of intuitionism that this would be a key constructive part of the theory being offered. Whilst admitting this Rawls indicates an ideal of "moral geometry" to which his "highly intuitive" approach aims as a kind of regulative idea.
The construction of the "initial situation" in terms of the "original position" is one that Rawls assumes can be justified despite the "highly intuitive" nature of the account given. The kind of justification involved is, however, very far from the notion of "moral geometry" that Rawls invokes as his ideal. The construction of the "initial situation" that is favoured as the "original position" is to be adopted as the one that "best expresses the conditions that are widely thought reasonable to impose on the choice of principles yet which, at the same time, leads to a conception that characterizes our considered judgments in reflective equilibrium". So there is a dual reason here suggested for the construction of the "initial situation" as the "original position". The first reason is that the "original position" relates "best" to conditions that we can see to be ones that we have good reason to impose on the choice of principles. The description of these conditions will thus form an essential part of the justification of the way the construction of the "original position" operates.
The second reason for preferring the construction of the "initial situation" as an "original position" is in terms of the consequences of so doing, namely that it produces a conception that shows how our considered judgments can be aligned by reference to "reflective equilibrium". So the principles by which the "original position" is constructed are ones that will characterize our considered judgments as a product of inter-actions that the position has modelled.
The general conditions of the "original position" being accepted as the best way of construing the "initial situation" have thus been presented. Rawls' next move is to articulate the presentation of alternatives that would be provided to the parties in the constructed situation. This move takes from us from general methodological preliminaries as we have been concerned with up to this point to a prolegomena to the construction of the position. This prolegomena has 2 parts, firstly one concerned with the alternatives that would be presented in any description of the "initial situation" and second an account of a set of constraints that would apply to the situation prior to the formal construal of constraints on the principles the situation should select. So the discussion of these "constraints" (what Rawls terms "circumstances of justice") are not yet the formal constraints that define the "initial situation" being determined as an "original position". Rather what sections 21 and 22 of Chapter 3 describe are what Rawls views as general characteristics of the "initial situation" prior to it being constructed as an "original position".
The account of a presentation of alternative principles of justice is first given with Rawls' earlier 2 principles already assumed to be present as ones available for those engaged in the choice situation. The alternatives that are presented within the choice situation are given in a "rough and ready" way that Rawls confesses is not itself satisfactory. The procedure operates by selecting a list of traditional conceptions of justice with the understanding that the construction of the situation as an "original position" will provide constraints that will determine a reasonable ground for the selection of principles.
The principles set out as open for choice are Rawls' 2 principles alongside four other sets of alternatives, one of which is, however, under the title of "mixed" conceptions, merely a way of combining Rawls' 2 principles with principles of "average utility" in certain ways. I won't dwell on these "mixed" conceptions at this point. The other 3 sets of principles are: 1) "classical teleological conceptions" which include the "classical" principle of utility, "average" utility and the principle of perfection; 2) intuitionist conceptions, pictured in terms of "balancing" principles or duties; 3) egoistic conceptions. The choice amongst these principles is understood, in the general sense of the "initial situation", to be highly guided by intuition in the sense of "balancing" considerations.
The final part of the "prolegomena" to the construction of the "original position" is by means of Rawls' account of what he terms "the circumstances of justice". These "circumstances" are understood as the "normal conditions" under which it is required that we cooperate with each other. As Rawls confesses, the conception of these "circumstances", whilst intended as broadly neutral prior to the construction of the "original position" is, nonetheless, accentuated in a Humean manner.
Essentially these circumstances are, however, characterized in ways that are also similar to the ones Kant describes in Idea for A Universal History as involving "conflict" between parties in addition to some "identity" of interests (or as Kant put it, "unsocial sociability"). The "conflict" concerns the desire each has for benefits for themselves, even at the expense of others and this produces the obvious basic problem of justice, that of "distributive shares". The basic requirement of justice is an answer to this problem, a problem that is effectively set then as part of the "initial situation".
The "circumstances of justice" is thus a grand name for a basic anthropological problem of access to resources in a way that distributes them in an acceptable and reasonable fashion. Amongst these circumstances are some that are "objective" and others that are "subjective". Amongst the "objective" conditions are the coexistence of people on a limited part of the earth, possessed of similar vulnerabilities and faced with general "moderate scarcity". The "subjective" circumstances concern the different priorities of people with regard to different personal projects and the shortcomings of knowledge and skill each suffers.
The general picture of the "objective" circumstances are routinely simplified as "moderate scarcity" whilst those of the "subjective" circumstances are "conflict of interests" though it is evident, particularly with regard to the "subjective" circumstances that these are, indeed, simplifications.
Finally, before closing this post, we should note that Rawls condenses the features of the "initial situation" by indicating that the circumstances of justice are known to the parties in this choice situation and that they have conceptions of the good that they wish to realise. Some questions about future generations are ventured here too but I will leave these aside in my account of the "initial situation" assuming at present that it is not obvious how much weight should be given to this. Part of Rawls' own description matches this when he states that the "initial situation" should not assume "extensive ties of natural sentiment". Finally, people in the situation are treated as having serious concern for their own interests. This completes the general description of the "initial situation" and allows the construction of the "original position" to be undertaken.