Irrational Beliefs that Are Psychologically Impossible to Hold
The title of this post might strike some as being rather peculiar. Does it make sense to claim that it’s irrational to have certain beliefs when it’s psychologically impossible to have them? Suppose I claim that the following is a rationality constraint on credences:
(1) If P(A if and only if B) = 1, then P(A) = P(B)
A reaction to my claim might be that if we really understand what `if and only if’ means, and assign a credence of 1 to `A if and only if B‘, it would be psychologically impossible for us to assign different credences to A and to B. Hence, or so one might argue, (1) is best interpreted not as a rationality constraint on our credences, but as a claim about our psychology.
Similarly, consider the following:
(2) Every spider is both an insect and not an insect.
One might think that if we really understand what (2) says, then it would be psychologically impossible for us to believe (2), and consequently, the question of whether believing (2) is irrational becomes senseless.
I don’t think that it’s psychologically impossible for us to violate (1) or to believe (2). But for the purposes of this post, I shall assume that it is, and argue that this neither prevents (1) from being a bona fide rationality constraint on our credences, nor (2) from being something irrational to believe. (more…)