Motivating reasons aren't necessary existents.
Propositions are necessary existents.
Motivating reasons aren't propositions.
This objection, the modal objection, seems to me to be pretty good. But, I can imagine a proponent of the view that identifies motivating reasons with propositions saying something like this. It's only when the subject has the right sort of propositional attitudes that the propositions can be the subject's reasons. This, I take it, is Miller's view from his recent Nous paper. Here's what he says:
Note that S’s bearing such an attitude towards p is itself a fact about p, and not a mental state. In addition, it is a fact which is not itself a part of the relevant motivating reason, but rather serves as one of that reason’s enabling conditions. Thus to use our previous example, the proposition there is widespread starvation in Iceland would not have served as one of my motivating reasons if I did not believe that there is widespread starvation in Iceland, even though strictly speaking it is the proposition which I believe rather than my belief itself that serves as my motivating reason (2009: 226).
I want to note two things. First, this response seems to concede that the motivating reasons that exist are necessary existents but then insists that it is contingent whether someone has a motivating reason and which motivating reasons they have. If it's intolerable for the motivating reasons themselves to be necessary existents, this response won't do. Maybe it's thought that it is tolerable for the motivating reasons to be necessary existents provided that the having of them is contingent. Note that what allows him to say that the having of them is contingent is the claim that it is only when we are in certain mental states that we have them and it is a contingent matter as to whether we are in such mental states. I think that's sort of right, but I think it's not going far enough.
It's clear that there's a difference between believing that the proposition p exists and believing p to be true. It's contingent whether someone believes any propositions to exist and it is contingent whether someone believes any proposition to be true. The question I have is simple. Why wouldn't knowing a reason to exist be sufficient for having the reason? If knowing a reason to exist is sufficient for having the reason, then I don't think that the account is going to get the modal profile of having a reason right. Here's why:
Knowing a motivating reason to exist is sufficient for having it.
Propositions are known to be necessary existents by the philosophically informed.
If reasons are propositions, the philosophically informed (who believe motivating reasons to be propositions) will know that they have all the same reasons in every possible world where they know propositions to be necessary existents.