I think Dancy (2000: 131) is wrong to say that we can give a non-factive explanation of an agent's actions. While Dancy seems to think this is coherent, this strikes me as contradictory:
(1) Mustard’s reason for running down the hall was that the murderer was chasing him, but of course he was mistaken about that.
It doesn’t sound like a contradiction to say things like, ‘His reason for Φ-ing was that p, but of course he was mistaken about that because owing to self-deception he thought that his reason was q.’ Dancy’s suggestion, however, is that (1) could be true and it could provide us with a non-factive explanation (i.e., a successful explanation that lacks a true explanans) when it is not true that the murderer was chasing him, not when the agent has a mistaken belief about his own motivations. If (1) is contradictory, then it seems (2) could be true only if there was indeed a murderer chasing Mustard:
(2) Mustard’s reason for running down the hall was that the murderer was chasing him.
If it's part of the story, however, that Mustard was mistaken in thinking that the murderer is chasing him and (1) is false, then (2) is false.
If (2) is false, you might opt for the view that says that Mustard’s motivating reasons are either mental states or the contents of his states. Miller, seems to opt for this sort of view. According to Miller, "considerations such as she loves me or I ought to keep my promise could ... be among the kinds of considerations which might motivate me to act if I happened to believe them" (2009: 250). He raises this objection to the view that identifies motivating reasons with facts:
For unless we are infallible about what facts there are, there will be plenty of instances in which we invoke motivating reasons in our practical deliberation and yet at the same time are quite mistaken about the existence of the facts to which they make putative reference (2009: 229).
When Miller says above that motivating reasons are considerations which might motivate an agent to act, he's taking considerations to be propositions rather than facts. Sticking with Mustard, I take it that on Miller's view (2) could be true even if:
(3) No one was chasing Mustard.
Again, it seems (to me) that (1) is false. It also seems to me that (1) is entailed by (2) and (3). Doesn't Miller's view face pretty much the same problem that Dancy's view faces?
Bracketing this problem, I'm not entirely sure what Miller's objection in the passage above is supposed to show. Mustard is fallible. That's obvious. He believes he's being chased, but he's not. I take it that Miller thinks that Mustard's fallibility concerns matters of fact but he's not mistaken about what his motivating reasons are. (If Mustard were mistaken about both matters, there's no objection to treating reasons as facts in the passage above.) Why should we assume that Mustard's beliefs about his motivating reasons are true even when the beliefs that figure in practical deliberation are false? It doesn't seem to me that Mustard can say truthfully, 'My reason for running down the hall is that there's a murderer after me'?