Beliefs are “afferent” mental states in the sense that facts in the world impose beliefs on our minds through our perceptions — the causal “flow” is inwards from world to mind. Desires are “efferent” mental states in the sense that ways we want the world to be impose themselves on the world through our actions — the causal “flow” is outwards from mind to world.
Because of beliefs’ dependence on the way the world happens to be, and because we’re mostly rational, we can often make people believe something whether they want to or not. We simply engineer a fact, or present them with evidence of a fact, and their beliefs duly re-arrange themselves to maintain their systematic function as a more or less accurate map of the world.
By sheer bad luck, a “rogue” fact can lead to a distasteful belief. So we simply cannot be held culpable for what we believe. We can have distasteful beliefs, but we cannot have blameworthy beliefs.
Desires are different. Together, our desires are more like a blueprint for the world than a map of the world. Through our actions, our blueprint for the world makes facts by re-arranging things in the world (as opposed to being sensitive to their prior arrangement, like beliefs). If we do something immoral, such as disregarding another person’s interests, we are blameworthy because our desires are bad. More specifically, our intentions are bad. Intentions are the heart of culpability.
As far as I know, Hume was the first philosopher to see that beliefs (“reason”) and desires (“the passions”) play complementary roles in causing actions, rather as orthogonal components can be used as a basis of a vector space. I think it was remarkably humane of Hume to see that none of us can be held culpable for our beliefs. They’re the “slave” of our “passions” — in other words, we use our “map” in order to realize our “blueprint”, so if things turn out nasty, blame the blueprint rather than the map. And beliefs are mostly a matter of luck anyway. As Hume’s predecessor Locke observed, a person’s salvation cannot depend on an accident of birth.
Many people think we can have racist beliefs. I think they’re wrong. ‘Racism’ is and should remain a word of condemnation. To preserve its ability to express condemnation and blame, we should strictly limit its application to bad motives, bad intentions — to the efferent mental state, desire — rather than to merely distasteful beliefs.