Why is this? The answer, I think, is that delusion is an attitude closely bound up with language -- an attitude sometimes called ‘acceptance’. To accept a proposition, in this sense, is to commit oneself to arguing for it, defending it, using in reasoning, and acting upon it. This is the attitude a scientist takes to a hypothesis, a lawyer to the claim that their client is innocent, a politician to a policy. It is a sort of intellectual commitment, which is active, reflective, and conscious. (For more on acceptance and a detailed presentation of the case for thinking that delusions are acceptances, see this paper.)
If delusions are acceptances, then several things follow. I shall mention three. First, delusions are active. Accepting a proposition involves making and executing a commitment to a certain pattern of argumentation and reasoning. Thus, deluded patients are actively engaging with their situation rather than just passively responding to it.
Second, delusion is motivated. We accept propositions because doing so answers some need or goal (not necessarily a conscious one). This might be simply the goal of accepting truths, but it could be pragmatic, as in the case of the lawyer. Thus, a new question arises about delusions: What need or goal does the formation of a delusion address? It could be epistemic; maybe the patient is just trying to make sense of their distorted experiences. But a delusion might serve other psychological needs, especially emotional ones. For example, it might be a comfort to have a simple framework for representing what is happening to one, however bizarre.
Third, delusions have limited behavioural influence. Acceptance does influence nonverbal behaviour -- the commitment extends to acting upon the accepted proposition -- but this influence is often limited. An acceptance may be deliberately restricted in scope (the lawyer accepts her client’s innocence for professional purposes only), and even if it isn’t, it won’t automatically guide routine behaviour. Acting upon an acceptance requires paying attention to what one is doing and deliberately regulating one’s behaviour to bring it in line with the proposition accepted. Moreover, even if one realizes that an acceptance dictates a certain action, one’s desire to adhere to it might be overridden by other desires. (Think of the effort involved in adhering to a moral principle one has accepted.) Thus, delusions will only influence behaviour that is under reflective control and may influence that only in a patchy way. This does not mean that a patient’s unreflective behaviour will be normal, of course. But it will manifest their distress in a raw uninterpreted form, rather than one constrained to reflect the pattern imposed by their delusion.
The idea that delusions are acceptances has been discussed before on this blog, but it has not received a lot of attention, perhaps because the literature on acceptance is not itself widely known. I hope this will change and that the hypothesis will be explored and evaluated in clinical contexts.