Chapter 15 of Parfit's On What Matters concerns contractualism and the first part of his re-working of the third 2002 Tanner Lectures which I treated previously. Contractualism is summarised by Parfit as based principally upon a claim to rational agreement concerning principles all ought to follow. However, whilst this point is relatively uncontroversial, Parfit quickly introduces a methodological reflection on the understanding of such rational agreement that is far from obvious. This is to the effect that the rational agreement in question would be one that produced outcomes that were expectably-best. In focusing on outcomes, and in relating to them in terms of what would be optimal, Parfit assumes that the best test for a contractualist view would be a consequentialist one.
Parfit further makes a second assumption that comes out in his very brief account of, and reply to, the work of David Gauthier. Gauthier's Hobbesian form of contractualism is faulted by Parfit for its basic use of a state of nature device. The real problem with this device, on Parfit's view, is that it concedes ground in advance to the inequality of power that exists between people and hence allows a kind of threat advantage to dictate the terms of the resultant agreement. This ensures that many "plain duties" as Parfit calls them are effectively flouted. So this second assumption is to the effect that where there appears to be an intuitive understanding of such "plain duties" that this needs to be accounted for on any plausible contractualist view.
Having these assumptions in hand Parfit turns to examining Rawls' form of contractualism though, notably, the way he approaches Rawls is very narrow. Parfit concentrates not on "justice as fairness" but rather only on "rightness as fairness", the general moral view that Rawls refers to a few times but never systematically developed. Part of Parfit's analysis of Rawls also works on the assumption that Rawls is committed to a desire-based view of reason, an assumption that seems to me faulty. It is not that Rawls assumes "desires" to be the basis of reason but that he does take it that there is a "thin theory" of what all people can be said to want which should guide elementary moral psychology.
Parfit looks at different models of the veil of ignorance that Rawls provides and finds fault with the thicker veil that Rawls appeals to. One of the reasons Parfit does this is distinctly odd, however, since he alleges that if there is a good basis for denying knowledge in the original position then such denial should apply as well even to the view that there are inequalities between people (of strength, intelligence, etc) at all. But the extension of the veil in this way has no practical purpose and even foils part of the point in invoking it. This would be the point of ensuring that the result of the procedure of construction of the original position would ensure that the design of the basic structure was most respectful of the worst off. This could not be achieved if we did not assume that there exists such a thing as a group that is worst off. The reason Parfit gets in to this discussion is that he has a prior agenda of bringing Rawls' view as close as possible to utilitarianism and in doing so he continues his methodological move of favouring axiological criteria of morality but this also cuts against the key constraints of the notion of right that Rawls builds into the original position.
These skirmishes with Rawls are still preliminary to Parfit's key discussion in this chapter which concerns what he terms the "Kantian Contractualist Formula", a formula specified as stating that everyone ought to follow the principles whose universal acceptance everyone could rationally will. This formula is evidently intended as a version of Kant's formula of the kingdom of ends. However Parfit cuts this formula off from the general discussions of contractualism in asserting that it is not based on achieving general agreement. It is instead motivated as a formula that is applied by individuals rather than a group so that the results of many over-lapping thought-experiments are meant to point to a generally agreed result. In arguing for this peculiar move Parfit draws on the Rawlsian argument against utilitarianism which alleges that the problem with the latter view is that it enables some to be sacrificed for the sake of the many. Parfit returns to the basic inequality between persons to reject the search for a rational agreement formula using the example of Gauthier to do so though this is again a pretty odd move given that the process of Rawls' original position does not at all model Gautheir's and shows that search for a rational agreement formula need not favour the strongest over the weakest.
More interestingly Parfit argues for a contractualist formula that does not suppose rational agreement is possible on the grounds that if unanimity is able to be achieved through a formula that is not based on presupposing it then it will be all the stronger an agreement. However what is assumed instead of such agreement is shared search by everyone for well-being that replaces the Rawlsian focus on primary goods.
Parfit proceeds to make two additional moves in the chapter. Firstly he introduces Thomas Scanlon's notion of contractualism according to which everyone should follow the principles that no one can reasonably reject. In introducing this formula however Parfit appears to move away from his earlier recognition of 'plain duties', a recognition that was used against Gauthier. This is done by denying that we should appeal to judgments about what is wrong in applying the contractualist formula and thus removes the ability to appeal to the 'plain duties' that were invoked against Gauthier. Further Scanlon himself states, and Parfit quotes him stating, that the problem with utilitarianism is that it produces results that are starkly at variance with our "widely held convictions", a move that supports the view that there is some ground for thinking that such convictions do affect the evaluation of contractualist formulas and are thus not irrelevant to their application.
In noting this point of Scanlon's Parfit again returns to the principal anti-utilitarian point made by Rawls to the effect that burdens imposed on some just for the benefit of others are prima facie to be rejected. Given judicious use of thought-experiements Parfit moves away from the strict version of this point and tries instead to restrict its application to special cases (such as describe how doctors should treat patients). In so doing the basic force of the principle is weakened.
Parfit's final move in the chapter concerns his implicit appeal to common sense restrictions on moral principles and these are now justified in terms of the view that contractualist principles are not themselves sufficient to describe wrong-making properties of actions but that they rather state higher-level properties of wrong-making that are intended to include the lower-level properties. This is meant to give us a reason not to include such lower-level properties in the application of the contractualist principle but rather in evaluation of its application. In making this large concession to intuitive views of what is wrong, however, Parfit also ensures that the examples to which he appeals are ones whose evidential force can be established, something of a tall order.