Martin Davies, Max Coltheart et al have in recent years outlined the 'two factor' framework for understanding delusions. This framework can seem so utterly reasonable that it might seem perverse or stupid to decline its offer of help in framing the question of how to grapple with psychotic thought. So of course that is just what I'm now going to try to do.
It would of course be more theoretically appealing, especially to fans of Occam's razor, to have a one factor theory. But then as Einstein supposedly said (along with all the other things he supposedly said): if you are out to describe the truth, leave elegance to the tailor. So perhaps it just is the case that a satisfactory explanation of why people have delusions will need to appeal to more than one explanatory pathological factor. Well perhaps, but let's see.
To start with let's recall why we tend to take ourselves to need two pathological factors. First the idea is that, to be deluded, you've got to entertain some rather bizarre idea. Maybe you've had some rather odd perceptual or bodily experiences. OK. But, moving on from the first factor, why on earth would anyone believe that such experiences were better explained by their accurately depicting how things are - rather than, say, by the fact that their perceptual or proprioceptive system had gone up the creek? And so the second factor is there to explain not why delusional beliefs are entertained but why they are then maintained. So there is not only some odd experience or thought process, but also some further odd habit of thought which secures the delusional consideration its home in the storehouse of the subject's beliefs.
So, what's to complain about? Well, here I want to suggest that this framework will only work if we start from the premise that delusions are failures or disturbances of the processes by which beliefs are formed and maintained. (As Davies has it, 'delusions are pathologies of belief'.) This may of course be true for some beliefs which warrant the epithet 'delusion'. However it is not, I suggest, true for those paradigmatic delusions of paradigmatically delusional 'illnesses' such as schizophrenic conditions. It is also not obvious, in the framework on offer, why such a delusional patient is at least seemingly capable of forming and maintaining other beliefs perfectly well.
I've addressed before this issue of schizophrenic delusional beliefs not being pathologies of believing. To recap: in my opinion, which I reckon is pretty much in line with clinical tradition, delusions arise not from a disturbance in the normal processes of belief formation/maintenance, but rather arise when reality testing (which is not hypothesis testing: that would be a failure of belief!) - ie the capacity to keep reality and fantasy apart - breaks down for some selective area of mental function. An imaginary product then masquerades, as it were, as a bona fide doxastic attitude. Or, to be clearer: the patient, I reckon, isn't involved in some second-order (metarepresentational) faux pas of mistaking their imaginings for their beliefs (Currie et al) - there is rather a more substantial first-order conflation of the being of these states which confounds our everyday delineations of them, and which is why we have the special qualifier of 'delusional' for delusional belief, delusional mood, and delusional perception. Furthermore, this process is often enough informed by all sorts of fairly everyday 'pathological' tendencies (confirmation bias, wishful thinking, etc), especially the tendency we have to bind intolerable anxiety by producing overly organised narratives.
So: the bottom falls out of your world, and an ersatz world is unconsciously invented to take its place. I'm not sure if this is two factors or one. It is certainly true that a rent in the ego may obtain without receiving a delusional patch. Let us put it this way: the form that delusion takes is a function of the rent; the content and the obtaining of the delusion a matter of the psychological needs of the individual. Not two discrete pathological causes then, not failures in two mechanisms, and not, I would urge, a failure in the normal processes by which beliefs are formed and maintained.