Lisa Bortolotti's Imperfect Cognitions blog has recently published a short summary that Bill Fulford and I wrote, advertising the view of delusions we took in our chapter in Schramme, T. & Edwards, S. (eds) Handbook of the Philosophy of Medicine.
I try to resist being drawn into substantial theorising but in this case I think a kind of non-theory theory might be called for. It also serves as a reason to connect delusion to tacit knowledge.
We think that there is a clinically recognizable local breakdown in that those relations which is not sufficient to take someone outside the space of reasons (a mere machine) but which resists specific understanding because, as Jaspers emphasised, the reasons cannot be shared. Further, just as rationality as a whole resists codification so breakdowns in the space of reasons do too. Recognising delusion is thus a matter for tacit rather than explicit knowledge.