Thoughts on Belief and Desire

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.

Still, suppose I'm wrong, and all desires take states of affairs, propositions, sentences, or some such as their accusatives in a manner roughly equivalent to the way beliefs do.  Evidently, some desires do so, at any rate, such as (say) Smith's desire that Jones be hired.  The relevant difference here (and I am much more certain of this than I am of my previous point) is that the latter are (likely, or at least pretty plausibly) closed under conjunction, while the former most assuredly are not: letting "Bel (p)" stand for "I believe that p" and "Des (p)" stand for "I desire that p",

Bel (p) & Bel (q) —> Bel (p & q), but

~( Des (p) & Des (q) —> Des (p & q) ).

The second thesis is, as I say, certain: it is obviously possible to simultaneously desire incompossible states of affairs to obtain, but absurd to desire their conjunction: for example, I might desire of a classmate Jane that she fail an exam (perhaps out of spite) while also desiring that she not fail the exam (perhaps out of friendship or natural sympathy), but this does not commit me to desiring that she both fail and not fail, which would be clearly irrational and absurd.  (The classic case of these sorts of incompossible desires would, of course, be Plato's Leontius, but similar scenarios are easily multiplied.)  I conclude, therefore, that, as desires lack an important structural trait of beliefs, and one crucial to the constraints of rationality on our doxastic attitudes, they cannot be to appetition as beliefs are to cognition.

I can't shake the feeling, however, that desires are roughly analogous to some sort of cognitive habit, so I'll end with suggesting a candidate for that role: finding plausible.  As with desires, we can find plausible several propositions the conjunction of which cannot be true, but to find a proposition plausible nevertheless in some sense sees it in a favourable light and could be taken as a reason for believing it, just as a desire is intelligibly a reason for an action.  But, in order to test and expand this thesis, I'd need to do more work and research on the logic of action and the role of plausibility in cognition, which is no mean feat.