Something which makes it hard to achieve conceptual clarity regarding the psychiatric distinctions of the organic, the psychogenic and the factitious is the perennial temptation to import metaphysical distinctions between the mental and the physical into their explication. Yet this is but a way philosophy hinders rather than aids psychiatry's reflective self-understanding.
One result of a glib importing of the metaphysical distinction into the psychiatric categories is a well-meaning psychologist's proffered: 'Oh, these distinctions between the organic and the psychogenic are so old-fashioned and dualist. These days we know we don't do well to assume that mental causes and neurological causes are two separate phenomena. The mind is not somehow some separate thing from the brain you know...'
The irony here is that it is the psychologist rather than the psychiatrist who is making the crass philosophical assumption - the assumption that in psychiatry the category of the 'organic' is to be understood in terms of that of the 'physical', the 'psychogenic' in terms of the 'mental'. The result is not that the distinction itself is shown to fail, only that the psychologist fails to achieve any rational reconstruction of it, and causes further muddle to boot.
So how are we to understand these distinctions? I propose that the fundamental category - the one we need to start with, the one an understanding of which is presupposed by the other categories - is that of the psychogenic. (Bet that surprised yer.)
A psychogenic condition, I suggest (tell me if I'm wrong!), is a breakdown due to intolerable pressure on the ego due to the thwarting of drives. Thus a breakdown due to unrequited love, to hopelessness, companionlessness, thwarted aggression, etc. Note that this definition says nothing about the degree of change in neurological structure or function. Perhaps a serious case of unrequited love involves all sorts of striking neurological alterations; I rather imagine it would! But the point of saying the category is fundamental is precisely to say that the nosology doesn't depend upon differentially diagnosing the absence of neurological alteration.
An organic condition, I propose, is one which is not psychogenically intelligible. (The concept of an organic illness in psychiatry is, I'm suggesting, not the same as the concept of an illness in general medicine, for the everyday concept of an illness is not left ontologically hanging on the absence of psychogenicity.) The alterations we find in the brain of an organically ill psychiatric patient (e.g. a late-stage syphilitic) are not due to thwarted drives (but to spirochetes!). Yet an organic illness is in one sense still defined functionally. That is to say, an organic illness is defined in terms of such impairment of an organ as causes disturbance in organismic (i.e. your) function. (If a spirochete eats away half your brain but gets so depressed with its miserable life that it gives up the ghost, and yet you suffer zero functional impairment, then in what sense are you ill? Who gives a monkeys if you've lost half your brain?) What is important, however, is that the illness is not constituted by an environmental thwarting of function. The brain changes are not caused by atrophy due to impossibility of drive satisfaction. Again, the concept of the psychogenic wears the differential trousers here.
A factitious illness is not an illness. It is someone pretending to be ill. You gotta understand it in terms of intentions to deceive.
We might add hysterical to the above list. If I had to define it I'd suggest: psychogenic with a form offering secondary gain (?is that right?). (And if anyone tells you that there is always and everywhere a clear distinction between intentional and non-intentional action, I recommend asking them where they got the license for their conceptual confidence. Our concepts so often have raggedy edges and admit intermediary cases. I for one see no grounds - other than egregious philosophical fiat - for saying that it must be the case that someone always either intends or does not intend something.)
It is natural for medics to worry that they may be mistaking psychogenic for organic illness. (The opposite of course may happen too, but is usually less fatal or less shaming for both patient and doctor - although let's not underestimate the disastrous existential and financial costs of undiagnosed or misdiagnosed psychogenic illness.) What can be said to reassure them?
On the one hand: not a lot. You just gotta do a careful examination of your patient. You can't just go around guessing that they have unfulfilled desires of such intensity as to cause functional breakdown. You look for the actual signs of this. But really this is just like the rest of medicine. Again: the category of the psychogenic is the one wearing the conceptual trousers here; the concept of the organic is only to be invoked when the former finds inadequate purchase.
Where I think the added pressure comes from is the supposed march of progress of neuroscience. ... 'We are finding out more and more about the brain, and as a result will find out that more and more which was previously understood to be psychogenic will be organic.' ... Yet regards that: how many disorders have actually been newly understood since neuroimaging came on the scene? We have learned something more about organic conditions like neurosyphillis, dementia, parkinsons, and epilepsy. Have we really learned any more about the causes of other mental illnesses from neuroscientific investigation? Most often we just learn something more about the typical neurological alterations in this or that psychiatric condition. Yet, once again, neurological changes in no way index organicity. To suppose they do is to collapse back into the metaphysical canard with which we started.
Nothing in neurology shakes the conceptual primacy of the psychogenic in psychiatry.