Belief, Certainty, and Doubt
Consider the following terms and their rough definitions:
A belief is a mental state such that, with respect to proposition p, one assents to p or takes p to be true. (It doesn’t follow that p is true!)
Psychological or subjective certainty is a mental state such that one is completely convinced that p is true. Psychological certainty is a property of the person who has it, not of the proposition he believes.
Epistemic certainty is a matter of having a belief which is infallible given the evidence that supports it. In other words, if a person is epistemically certain that p, then it is impossible for that person to be wrong that p given his evidence for p.
Doubt is a mental state such that one lacks psychological certainty about p.
Note that (a) belief is necessary but not sufficient for psychological certainty. If you are psychologically certain that p, then you believe that p. For example, if Detective Smith is (subjectively) convinced that the butler committed the crime, then Smith believes that the butler did the deed.
Yet (b) one can believe that p and lack psychological certainty that p. (The same holds for the relation between belief and epistemic certainty.) For example, meteorologist Jones might believe that there will be rain tomorrow and not be psychologically certain about the rain.
It seems to follow from (b) that (c) belief is consistent with doubt; one can believe and yet doubt the truth of p. For instance, Green might believe a political claim and yet have some degree of doubt that the claim is true.
It’s also important to recognize that (d) psychological certainty is inconsistent with doubt; one can’t be psychologically certain that p and yet doubt that p. However, (e) one can have epistemic certainty that p and yet lack psychological certainty that p. For instance, Brown might possess epistemic certainty that some mathematical proposition is true and yet lack psychological certainty about the matter, perhaps because he does not realize that he has epistemic certainty about the mathematical proposition, or perhaps because he suffers from anxiety and thus lacks psychological certainty although he realizes that his acceptance of the proposition is highly justified.
It seems to follow from (e) that doubt is consistent with epistemic certainty. One can have an infallible belief about, say, the truth of some logical entailment, and yet because of an emotional problem (say, extreme anxiety concerning a logic test) harbor doubt about the entailment. Ideally, however, one who has epistemic certainty also has psychological certainty.
Lastly, note that (f) it is possible for one to be psychologically certain that p and yet p is false. Hence, such a person would be psychologically certain but wrong that p. For example, Jackson might be psychologically certain (i.e., totally convinced) that Bigfoot exists and yet wrong.
What’s the practical significance?
For one thing, belief does not require psychological certainty. For instance, one can believe a religious claim and thus count as a believer, and nevertheless lack psychological certainty for that claim.
For another, it’s possible to believe a claim and doubt it. One can believe the religious claim but have some degree of non-rejective doubt about it. In fact, doubt can motivate rigorous intellectual inquiry, which can increase one’s degree of conviction that the claim is true. Inquiry can also decrease one’s degree of conviction, depending on the belief, the evidence for it, and how far one takes the inquiry. In short, doubt is not necessarily a bad thing, nor is it necessarily opposed to belief, although some speak of doubt in negative terms.
For a third, one’s psychological certainty about a claim is no proof that one has a true belief about that claim. A politician might claim (subjective) certainty that his plan to solve a national problem will work, and yet the plan fails. An automobile mechanic might claim (subjective) certainty that your car repair will be done in two hours, and yet you are still waiting four hours later. Your teenaged son might say that he’s (subjectively) certain that he’ll get an A on Friday’s test, and yet …