  The will, Hume claims, is an immediate effect of pain or pleasure (T 2.3.1.2) and “exerts itself” when either pleasure or the absence of pain can be attained by any action of the mind or body (T 2.3.9.7).  The will, however, is merely that impression we feel when we knowingly give rise to an action (T 2.3.1.2); so while Hume is not explicit (and perhaps not consistent) on this matter, he seems not to regard the will as itself a (separate) cause of action.  The causes of action he describes are those he has already identified: the instincts and the other direct passions.