Irrationality is considered a defining feature of delusions in many influential definitions. But in what sense are delusions irrational?
Delusions can be procedurally irrational if they are badly integrated in one’s system of beliefs. They can also be inconsistent with other beliefs one has. Lucy who has Cotard delusion believes at the same time that dead people are motionless and speechless, that she can move and talk, and that she is dead. Here there is an apparent inconsistency that is “tolerated”, that it, doesn’t lead her to revise or reject one of the beliefs. Typical delusions are epistemically irrational, that is, they are badly supported by the evidence available to the subject, and they are not adequately revised in the light of new evidence. John who suffers from anosognosia doesn’t acknowledge that one of his legs was amputated and explains the fact that he can’t climb stairs any longer by a violent attack of arthritis.
These examples are striking. For many philosophers, the irrationality of delusions is a reason to deny that delusions are beliefs. Lucy can’t really believe that she’s dead, maybe what she means is that she feels empty and detached, as if she were dead. John can’t really believe that he has both legs because there is no problem with his visual perception. Maybe he wishes he still had both legs. This way of discounting delusions as metaphorical talk or wishful thinking is appealing. It is based on the view that there is a rationality constraint on the ascription of beliefs. We wouldn’t be charitable interpreters if we ascribed to Lucy the belief that she’s dead and to John the belief that he has arthritis.
I want to resist the idea that people with delusions don’t mean what they say. First, people often act on their delusions and base even important decisions in their lives on the almost unshakeable conviction that the content of their delusions is true. We couldn’t make sense of their behaviour at all if we couldn’t ascribe to them delusional beliefs. Second, the type of irrationality delusions exhibit is not qualitatively different from the irrationality of ordinary beliefs. Delusions may be irrational to a greater extent than ordinary beliefs, and the examples we considered were certainly puzzling, but procedural and epistemic irrationality can be found closer to home.
Students believe that wearing clothes of a certain colour will bring them good luck during the exam and nurses believe that more accidents occur in the nights of full moon. These beliefs are certainly inconsistent with other beliefs well-educated people have about what counts as the probable cause of an event. Prejudiced beliefs about black people being more violent are groundless generalisations that can be just as insulated from evidence and as resistant to change as clinical delusions.
Maybe what makes delusions so puzzling is only that they are statistically less common (not necessarily more irrational) than other procedurally and epistemically irrational beliefs.