A has a reason to φ iff A has some desire the satisfaction of which will be served by his φ-ing.
There is a reason for A to φ iff there is some desire the satisfaction of which will be served by his φ-ing.
Sentences of the forms 'A has a reason to φ' or 'There is a reason for A to φ' (where ‘φ’ stands in for some verb of action) seem on the face of it to have two different sorts of interpretation. On the first, the truth of the sentence implies, very roughly, that A has some motive which will be served or furthered by his φ-ing . . . On the second interpretation, there is no such condition, and the reason-sentence will not be falsified by the absence of an appropriate motive. I shall call the first the 'internal', the second the 'external', interpretation. (Given two such interpretations, and the two forms of sentence quoted, it is reasonable to suppose that the first sentence more naturally collects the internal interpretation, and the second the external, but it would be wrong to suggest that either form of words admits only one of the interpretations.)
If something can be a reason for action, then it could be someone’s reason for acting on a particular occasion, and it would then figure in an explanation for that action.