Bernard Williams, answered from the texts and cited to the page.
A reason for action must be capable of motivating the agent given their existing motivational set S, through some sound deliberative route.12 There are no "external" reasons — reasons that are supposed to bind the agent regardless of what they actually want, what they care about, what their projects are. The "external reasons statement," when offered as a substantive claim about the agent, is at best a moralistic remark — a claim about what the agent should desire, given the speaker's values, dressed up in the language of reasons.3
It has no leverage on the agent who does not desire it; the agent is not irrational for not being moved by it. The internal-reasons thesis is not a form of crude Humean instrumentalism (the agent is not restricted to what they presently happen to want; the deliberative route can include imagination, information, and reflection that modifies S in significant ways), but it preserves the basic insight that motivation cannot be conjured from outside the agent's actual life.
Korsgaard's Kantian "constitutive reasons" are external reasons under another label, and the same objection applies. Parfit's reasons-fundamentalism (the view that some reasons are irreducibly normative facts independent of the agent's motivational set) takes the external-reason claim to the metaphysical level; my objection to it is the original one.45
What is it that one comes to believe when he comes to believe that there is reason for him to 4, ifit is not the proposition, or something that entails the proposition, that if he deliberated rationally, he would be motivated to act appropriately?Moral Luck, p. 124
The extemalist view is that this is not a necessary condition, and that it can be true of A that he has a reason to cl> even though A has no motivation in his motivational set that could, either directly or by some extension through sound deliberation, lead him to cj>. It is a further question whether the formulation provides a sufficient condition of an agent's having a reason to cj>.Making Sense of Humanity, p. 35
Any of these can be sensible things to say. But one who makes a great deal out of putting the criticism in the form of an external reason statement seems concerned to say that what is particularly wrong with the agent is that he is irrational.Moral Luck, pp. 124–125
However, he calls someone who lacks such desires ' a wanton', and talks about him as though he were a certain, undesirable, sort of person. For comment on Frankfurt, see Watson, 'Free agency'. 14 There is fuller discussion of this in Shame and Necessity, chapter 3.Making Sense of Humanity, pp. 33–34
In short, the questions of responsibility, in the sense of what the law's response should be to these happenings, are questions about the state and the intentions of Caligari and of Cesare, and of the relations between those people. It is not a question that can be answered by ascribing to one of them and not to the other actions and intentions which are the cause of death.Philosophy as a Humanistic Discipline, pp. 109–110
"Internal and External Reasons" (1980) is the founding text of the modern internalism/externalism debate in ethics. Korsgaard developed the central Kantian reply (constitutive reasons of agency); Parfit's On What Matters (2011-2017) is the most sustained externalist project of the late twentieth and early twenty-first centuries. Scanlon's reasons-fundamentalism is a different externalist position, less Kantian, more contractualist. McDowell's "Might There Be External Reasons?" defends a position closer to Korsgaard's; the contemporary sub-Humean tradition (Schroeder, Setiya in his earlier work) has refined internalism in non-Humean directions. The debate is not settled; the basic Williams thesis remains a live position with sophisticated defenders.