Beliefs are not to cognition as desires are to appetition, or at least they aren't obviously so.
Firstly, beliefs seem generally to take propositions or the like as their accusatives (I believe that p), whereas desires needn't always or for the most part do so. I can, for example, be said desire a building, person, or meal in a way I can't sensibly believe a building, person, or meal; try saying so, and I end up with a different sense of belief, one where I trust the testimony or say-so of my object. An illustration of this difference would be that, for many desires, one can sensibly ask, "How much does what you desire weigh?" whereas for no beliefs (excepting the special testimony-involving sense just mentioned) would the corresponding question "How much does what you believe weigh?" make any sense. This might be a merely linguistic point, but I'm inclined to think otherwise.