Full Idea
A desire cannot be identified with a disposition to act, since the agent might not know that a particular act leads to the thing desired, and thus might not be disposed to do it.
Gist of Idea
Behaviour requires knowledge as well as dispositions
Source
Ned Block (Troubles with Functionalism [1978], p. 69)
Book Reference
'The Philosophy of Mind', ed/tr. Beakley,B /Ludlow P [MIT 1992], p.69
A Reaction
One might have a disposition to act, but not in a particular way. "Something must be done". To get to the particular act, it seems that indeed a belief must be added to the desire.