My belief that behind the purple door there is a raging tiger who would painfully kill me in a way that is very bad for me is sufficient to motivate me not to open the door. According to Hume, one needs to add to this belief the desire not to be painfully killed. Adding such a desire on top of the belief seems quite unnecessary.
What, then, do I make of the apparent possibility of Simone's believing that behind the purple door there is a raging tiger who would painfully kill her in a way that is very bad for her, and yet her being entirely unmoved by this belief? It seems that this possibility implies that there must be something more than the mere belief that is motivating me, since Simone has the belief and is not motivated. But this reasoning is fallacious. That there are cases where an A-type event does not cause a B-type event does not imply that something more than an A-type event is needed to cause a B-type event. It could, instead, be that there is something more in the cases where the A-type event does not cause a B-type event, for instance some defect that blocks the A−B causal pathway.
What explains that Sally the Sheep is four-legged is just her being a sheep. Her mate, Rob the Ram, is three-legged, although he is just as much a sheep. We do not need to posit a further cause of Sally's four-leggedness beside her being a sheep. Rather, we need to posit a further cause of Ram's three-leggedness, such as genetic damage or an accident with some equipment in his woodworking shop.
Typically, when A-type events normally cause B-type events, a sufficient explanation of a B-type event in normal circumstances need only cite an A-type event. But when there is a departure from the normal, something more needs to be cited. Now it is clear that my being motivated by my belief is the norm, while Simone's case is abnormal. Thus, rather, it is Simone's case that calls for a further explanation, not mine.