The first part of the first volume of On What Matters is concerned with the understanding of reasons and Parfit's first chapter concentrates, in particular, on normative concepts. Since the point of the first part appears to be to provide a theory of reasons, it is not that surprising that the "normative concept" that the first chapter is particularly concerned with is reason itself.
Reason is looked at in terms of what it means to say we have a reason to believe something to be the case. So, reason is not looked at as a general capacity (by contrast, say, with understanding) but rather in relation to what is meant when we make claims about having "reasons". The theory concerning "reason" is hence meant to arise from the practice of indicating some things as acting for us as "reasons". So a reason, in this general sense, is a pro-attitude of a special sort.
However, whilst this appears to capture Parfit's general account of "reasons" there is an immediate complication since we can have "reasons", on his view, that we are not aware of having. What this means is that "facts" of certain sorts act as reasons for acting in certain ways to us even though we are not aware that they do so. In this respect, the "reason" in question indicates what we should do if we were aware of its force.
Reasons are capable of weighing in a cumulative way since there can be a number of reasons to act in a certain way and the more that point towards this kind of action the more reason there is to act in that way. This eventually reaches the point that some reasons are then declared decisive and if they are much the strongest reasons of all we can state they are strongly decisive.
Naturally the situation of responding to reasons is often not like this since there are many cases where reasons to act in different ways seem to weigh in reasonably similar fashions. (Though Parfit views this situation as an "imprecise" one.) Facts can, further, give us reasons to ignore some reasons. A decisive reason to act in a certain way provides us with the conclusion that we should or ought to do something and this gives us a decisive reason-implication. The general sense of "ought" (whether used morally or not) indicates that someone (whether ourselves or another) takes there to be here a case where we have such a decisive reason-implication.
Having looked initially at "reasons" Parfit subsequently turns to "rationality". It is interesting that the account goes this way rather than starting with rationality and providing an account of "reasons" on its basis. The latter approach would take "reasons" to be a general product of "rationality" whereas Parfit, by contrast, assumes "rationality" indicates a particular sort of response to "reasons".
Again, however, the view of "rationality", whilst it implies something more general than the notion of "reasons", is still not used in reference, in the first instance, to a general theoretical account. Rather, again, Parfit opens by looking at a practice of use of reference to "rationality", aligning the term with what is involved in our declaring something to be "reasonable" or "sensible" and its converse, "irrational", with what is involved in declaring something "stupid" or "crazy".
In choosing between different acts there can be a number of facts that give, in a general way, reasons to act in specified ways and these are termed by Parfit, relevant, reason-giving facts. What we ought rationally to do depends on the beliefs we have concerning these facts. "If we have certain beliefs about the relevant facts, and what we believe would, if it were true, give us a reason to act in some way, I shall call those beliefs whose truth would give us this reason." (34)
A "rational" belief can be comprehended on the basis of this discussion of how relevant, reason-giving facts, correctly construed, operate. So an act of ours is rational if the belief we have concerning the facts relevant to this action would give us sufficient reason to act in this way. We ought rationally to act, in certain ways, by contrast, if the reasons for acting in these ways are decisive.
Given that the facts in question are ones we may or may not be aware of it turns out that what we ought to do turns a lot on what we would do if our epistemic condition was rightly attuned. Parfit states that false beliefs, whilst able to give people reasons to act in certain ways, lack normative force as they do not count in favour of the actions in question (though this is not clearly his decisive view since he goes on to say that there is a "normative force" in these situations when people believe they have a reason to act in a certain way even though this is not a "real" reason so to act).
When we are trying to decide what to do, the key question for us is the reason-giving facts but when we ask if someone has acted rationally we are rather asking whether they deserve commendation or blame for acting as they did. However, this is only partially true since possession of false beliefs might indicate that others have acted in ways, that, given their beliefs, were not irrational.
When we have a reason to act in a certain way and we act in accordance with the reason we have adopted something as a motivating reason. Our moral reasons for action are not equivalent to decisive-reason-implications as we can have decisive reasons that are not moral and we may find that, despite thinking some action is the moral one to perform, we still have not been given decisive-reasons that imply we should act in this way. (This point suggests an admission of the case of the moral sceptic.)
Parfit turns finally to the question of what is involved in declaring something to be "good" and, again, this is viewed in relation to the practice of the use of the term in reference to reason-implications. So a "good" something is one that possesses properties of a sort that give us a pro-attitude towards it and hence declarations of something as "good" give us a "reason" to adopt this attitude. This is particularly relevant in relation to descriptions of things as "good for us". In thinking of the "good" in this way Parfit has moved to a default position of relating to this notion in terms that reflect a sense of "well being" so it is unsurprising that hedonic considerations are referred to at this point. However, the chapter concludes with a contrast between the self-interested view that is at work in considering our own well-being to the "impartial" point of view at work in thinking of general well-being. That the chapter concludes with these descriptions of the "good" does, however, already appear to weight discussion towards an approach of a particular sort when we reach the more substantive ethical accounts we expect later in the work.