Parfit has a habit of referring to Henry Sidgwick's Methods of Ethics as a "great, drab book" though the use of the term "drab" applies to some of Parfit's own discussion in Part 1 of On What Matters as Chapter 5 adds very little to the discussion of Chapter 4. Due to this I am turning in this posting to a response to Chapter 6 in which Parfit addresses the theme of the "dualism of practical reason", a topic first raised by Sidgwick.
This topic of the "dualism of practical reason" concerned, for Sidgwick, the contrast between rational claims of impartial reasons for action and rational egoistic claims. It is also stated by Parfit in more directly moral terms as a conflict between duty and self-interest. Sidgwick took the contrast between these two types of reason or moral claim to be a serious problem as he appeared not able to find some neutral third view that allowed for adjudication between their claims. Parfit describes the presence of this problem in Sidgwick as due to Sidgwick believing that there are here two viewpoints. Parfit presents an account of why we should reject the two viewpoints position and this account has, in turn, been critically assessed by Michael Smith.
Parfit's reply to the conception of "two viewpoints" appeals to two main claims. The first is that when we are trying to decide what to do, we should, according to Parfit, ask this question "from our actual point of view" not from an "imagined" impartial point of view. We don't need, says Parfit, to compare the contrast between partial and impartial reasons from a third, neutral point of view as our actual, present point of view is sufficient. What makes it sufficient? After all, according to Sidgwick, such a "present" point of view seems to presuppose the egoistic conception and thus merely beg the question between partiality and impartiality. However, Parfit denies this claim pointing out that we can, and often do, prefer an "impartial" standpoint in actuality rather than defaulting automatically to an egoistic standpoint. Further, "we have such impartial reasons even when our actual point of view is not impartial".
Now Chapter 6 does not resolve the "dualism of practical reason" though it is evident that Parfit is aiming to resolve it later. One of the ways it is intended to resolve it is by reference to a notion of "goodness" that Parfit mobilises and which was mentioned already back in Chapter 2. This is "goodness" in the "reason-implying sense" which implies that that there are "certain kinds of facts" about something's nature or properties that would, at least in certain situations, give us strong reasons to respond to it with the appropriate pro-attitude. However, this kind of goodness was contrasted, already in Chapter 2, with the notion of "goodness-for" which indicated something as "good" in relation to our sense of our well-being.
This contrast between two types of "goodness" is related by Michael Smith to a broad characterisation of Parfit as a "consequentialist" where all that Smith means by this is that Parfit thinks that "we can analyse all facts about what we have reason to do in terms of facts about the value of the outcomes of the things that we can do". This is described by Smith as a "weak" sense of "consequentialism" as it makes no assumptions at all about what is of value including making no assumption to the effect that all values are "impartial". Smith proposes that this "weak" sense of "consequentialism" is the right one by which to characterise Parfit's general view since, in making no such specific assumption about "value" it permits the distinction between two types of "goodness" that Parfit has made in Chapter 2.
However, Smith's challenge now asserts itself, as, in saying that the "weak" sense of "consequentialism", added to the two senses of "goodness", ensures that Parfit's view replicates the "dualism of practical reason". Given the distinction between the two types of goodness it appears that there exists precisely the incommensurability that bothered Sidgwick so that there is not available any means of resolution between the two "viewpoints" they appear to represent.
Now, just on the level of Chapter 6 alone, it is evident that Parfit provides no "resolution" of the "dualism of practical reason". However, there are ways in which he attempts to mitigate it and it is worth reviewing these to see if they help to reduce the difference of "viewpoint" that, on Smith's account, resurfaces in Parfit's characterisation so widely that we have here merely another way of stating the "dualism of practical reason". Well, firstly, as we saw, Parfit takes it that there is no need to search for a "neutral", third, point of view separate from the "standpoints" assumed to exist by Sidgwick. The reason for this is that Parfit argues that our "actual" reasons will be sufficient, something that Smith finds mysterious. Why does Parfit assume that this appeal to "actual" reasons helps at all?
The basic ground for this appeal to "actual" reasons is simply that the situation in which the distinction between partial and impartial reasons manifests itself for us as one in which we have some kind of dilemma is also one in which "reasons" are present in the "facts" in question. This appears then to invoke the "weak" sense of "consequentialism" Smith mentioned as a way of indicating that the "situation" of the judgment is one that speaks to us through the analysis that the "consequentialist" assumption would make. Why does this not, however, simply beg the question against the egoistic view, the one at work in "goodness-for" being assessed as important?
One of the reasons why Parfit seems to think not is due to the next way he attempts to mitigate the conflict which is through introducing "close ties" between ourselves and others, ties that involve us taking the well-being of some others to have special importance. This helps to show that it is not evidently true that the "actual" standpoint has to be one in which egoistic reasons are taken to be supreme but how does that help with the difference between the two types of "goodness"? Perhaps it indicates that what is "good-for" me may include some things that are also "good" in the other, "reason-implying" sense so that these two forms of "goodness" need not be seen, as Smith assumes, as necessarily divergent from each other. This may produce odd results as when we take it to be "good-for" us to promote the interests of someone else above our own interests but isn't this common with, for example, parents in relation to children? This kind of example helps to show that "goodness-for" is not necessarily equivalent to egoistic goodness and so that Parfit's difference between kinds of "goodness" is not as obviously as Smith assumes a replica of the "dualism of practical reason" in other terms.
Parfit's general strategy up to this point in the book has been to try to defend the conception that the "dualism of practical reason" can be defeated by showing that reasons are such that we can, and do, respond to features of "intentional objects" in a way that manifests our concern with their intrinsic properties. I have suggested that most of his arguments to show this have failed as he works with dual assumptions about the divide between "subjective" and "objective" reasons that cut against his arguments. On the one hand he takes Kantian views about reason to be "subjective" and yet, on the other hand, most of his arguments against the "subjective" view of reasons turn on appeals to claims about desire-based reasons (precisely not applicable to Kantian views). As a result Parfit conflates Humean and Kantian views about reasons and thereby does justice to neither.
However, whilst this general strategy is problematic the desire to locate a view of reason that enables practical reason to be understood in such a way that the move beyond partial reasons to universal ones can be made is commendable and in line with a Kantian impulse. When Parfit comprehends the "two" types of "goodness" in the way he does it is not obvious to me, as it seems to be to Smith, that he thereby simply replicates Sidgwick's dualism. Whether he can resolve the "dualism" turns precisely on how he goes on to comprehend the rational structure of morality and Chapter 6 just starts the turn towards this task. It is from this point on that we should evaluate the success or otherwise of the book.