Bernard Williams begins an influential essay  by defining internalism about reasons as follows:
Internalism There is a reason for person A to φ only if A has some motive which would be furthered by his or her φ-ing.
Plenty of philosophers have found something intuitive about this idea, but there has also been no shortage of disagreement over the exact sense in which A must “have some motive” which φ-ing must further. In the introduction to a recent anthology of literature on internal reasons, Kieran Setiya  seems to think that the most attractive versions of internalism are those which satisfy the explanatory constraint. Bernard Williams gives it best:
EX If something can be a reason for action, then it can be someone’s reason for acting on a particular occasion, and then it would figure in an explanation of that action. (p. 106)
There are at least a few reasons for adhering to EX. You might think that there is a unified account of explanatory and normative reasons, and that EX is a link in that unification. You might think that what it is for A to have a motivation which would be furthered by A’s φ-ing is just for there to be some p such that A is disposed to make p A‘s motivating reason for φ-ing. If you’re inclined to believe either of these, you’ll probably think with Setiya that the broadest plausible version of internalism is IR:
IR The fact that p is a reason for A to φ only if A is capable of being moved to φ by the belief that p. (p. 4)
However, I don’t see how EX could possibly be true, as I don’t think it can overcome the kinds of cases which motivate so-called “advice models” of reasons. I argue that an agent can have a reason for action which, qua reason, could not possibly motivate them. This undercuts the motivation for thinking that IR is the correct way of understanding internalism.