What is Belief?
Can belief be defined? A prima facie attractive idea is that belief is to be defined as assent: to believe that p is to assent to a sentence s that means that p. Thus I believe that sharks bite if and only if I assent to “Sharks bite”. The trouble with this is that there can be insincere assent: I say yes to the sentence but I don’t really have the corresponding belief. You can clearly assert something you don’t believe. It might be replied that the assent has to be sincere: but that can only mean that I must believe that the sentence to which I assent is true, precisely because I believe that sharks bite. But now we have presupposed belief in our analysis of belief. Nor does it help to push the sentence inward, as with the language of thought: the assent will only work if it is believed that the inner sentence is true. So the analysis is circular.
Another idea (associated with Ramsey) is that belief can be defined as what you are prepared to gamble on: you believe that p if and only if you would be prepared to gamble on the truth of p. More generally, belief is reliance—you rely on the truth of p in your actions. A belief is “a map by which we steer”, as Ramsey says. That sounds right—we do rely on our beliefs in acting—but it surely gets things the wrong way round. It is true that belief steers action, but only because it is the cause of action: you act as you do because of your beliefs—it is not that you have those beliefs because of your actions. I avoid sharks because of my belief that sharks bite; it isn’t that I believe that sharks bite because I avoid them. That is suspiciously behaviorist, as well as conceptually backward. Moreover, is it really true that belief is logically impossible without the ability to act? Can’t I have purely theoretical beliefs?
Is belief then indefinable? Is the concept of belief primitive? That is a tempting conclusion, but let us be patient. We may not be able to define it by moving outside of its conceptual territory, trying to get external leverage on the concept, but it might still be definable in some weaker sense. It may have conceptual joints or liaisons. Note first the connection to commitment: if you believe that p you are committed to the truth of p. But what does that mean? It means that you are reluctant to accept that not-p. You are not agnostic but committed, so it is harder to convince you that not-p than it would be if you were neutral. So belief is essentially resistance to contrary evidence or argument—that is, it is harder to convince someone to abandon their belief that p in favor of not-p than it is to convince them to believe that not-p given prior neutrality. It is harder to convince someone who is a theist to be an atheist than it is to convince someone to be an atheist who is an agnostic–or the other way round. Belief is reluctance to believe the negation. Belief is commitment to p as opposed to not-p. A true believer is someone who stands by her proposition—who is “faithful” to her proposition. As fidelity is reluctance to stray, so belief is reluctance to cognitive change.
Interestingly enough, desire has a similar connection to reluctance. If I desire something, I am reluctant to do without it: I want the thing in question and I want to avoid not having it. To desire something is to be unwilling to lack it: you cannot desire something and yet be perfectly content not to have it. So there is something negative about desire—its intentionality refers to a lack. To desire X is to be averse to the lack of X. There is thus a formal symmetry between belief and desire, since belief too has negative intentionality: to believe that p is to reject not-p—to be unwilling to accept that not-p. In both cases the state of mind includes a positive and a negative component: p and its negation, X and its lack. Both these components constitute the mental states of belief and desire—pro one thing and anti another.
If this is right, reluctance is a deep trait of the mind—that is, of a mind containing belief and desire. The states of belief and desire entail patterns of reluctance in the mind. We might even say that belief-desire psychology is a theory of mental reluctances, since reluctance is constitutive of their nature. When our ancestors started noticing patterns of reluctance in each other they invented the concepts of belief and desire to sum up those patterns: if someone is unwilling to do without X, then they are said to desire X; and if someone is unwilling to accept that not-p in the face of evidence and argument, then they are said to believe that p. Folk psychology is a theory of reluctance patterns—psychological resistances. Psychologists speak of approach and avoidance behavior: belief and desire involve both approach and avoidance. To be enthusiastic about something is to be averse to its opposite. Belief and desire involve dispositions to resist, rationally or irrationally. The stronger the belief or desire the stronger is the resistance.
This means that belief has a conceptual connection to the will, as has desire. To be sure, we do not will to believe; but we are always to some degree unwilling to abandon our beliefs. Belief is a close cousin to dogmatism. It is not that people are always rigidly opposed to abandoning their beliefs in the face of counterevidence—though one might be forgiven for supposing this to be a universal human trait–but they are always less prepared to change their mind if it is already made up. The difference between the agnostic mind and the committed mind is precisely that the latter is harder to shift. That is what belief is. So the will comes in at that point: we are unwilling to make a mental change once we believe something, compared to the state of agnosticism. When we are very unwilling we may be accused of dogmatism, but it is built into the nature of belief that it entails resistance to change. If you are totally convinced of something, it will be very hard to bring you to abandon your belief—you will fight tooth and nail to hang onto it. Belief is thus definable as reluctance to accept belief revision. 
 Knowledge might then be (partially) defined as justified reluctance to accept the opposite—being justifiably disinclined to change one’s mind. Knowledge is having good reasons to stick with what one believes in the face of alternatives.