Shah (following Williams) holds that:
1) R is a reason for X to Φ only if R is capable of being a reason for which X Φs.
Suppose that God will reward people who act from virtuous motives. This is clearly a reason for them to be virtuous. But it's not a reason that they can recognize, or act upon, because in doing so they would be acting from self-interest rather than virtue. So they wouldn't get the divine reward. Thus it would be self-defeating to act for this reason. The continued existence of the reason is dependent upon its not being recognized. Once it is recognized, the reason will disappear. Nevertheless, it seems clear that, so long as nobody is aware of it, the divine reward is a reason for them to act virtuously. So (1) is false. Just as there can be unknowable truths, so there can be inaccessible reasons, i.e. considerations that count in favour of an action even though they cannot possibly "guide" it.