As ought to be well known, psychologists are prone to conflate various different senses of the verb 'understand' when considering the question of the intelligibility of schizophrenic delusions. I'll outline a few forms and then consider their in/compatibility.
Thus we have an ordinary sense of 'understand' where I come to fathom what someone says or does in coming to see their reason for thus saying or doing - the end (Aristotle's 'final cause'), say, at which that speech or activity aims. It is this kind of understanding, surely, which Jaspers has in mind in talking of the un-understandability of the primary delusions of the schizophrenic person. Try as I might I can't make the reasons they proffer my own.
And we have another kind of 'understand' in play when I grasp, if it were as much as possible to do so, what neuropsychological mechanisms had temporarily become disturbed when someone is deluded. Perhaps in offering a causal explanation of this kind I can be said to be fathoming the 'material' or 'formal' causes of the change in psychological functioning that constitutes or underpins delusionality.
A further kind of 'understand' is in play when we think of how, say, a paranoid delusion may bring at least initial relief to its sufferer, by systematising, concentrating and (in an unhelpfully vague use of this term, which is at least a little less misleading than the 'explaining' function which the cognitive psychologist often posits as the cause of delusion formation) 'rationalising' the unbearable antecedent psychotic terror. It isn't exactly obvious to me which level of explanation this form of understanding is best pegged to. For example should we think of it as primarily referencing the subject's motivation - that the subject's forming and maintaining their delusion is motivated by a drive to reduce selfhood-disrupting terror? Or should we think of it in terms of the subpersonal lure of, as it were, an 'attractor basin' in the systemic dynamics of the brain's function - a replacement form of inner equilibrium which can kick in when equilibrium becomes hard to come by in the context of what might normally be those sustaining reality-contact-promoting brain-body-world-other-afferent-and-efferent feedback loops (certain of which have for the delusional subject become radically destabilised) - a replacement form, that is, characterised by an intra-neural short-circuit. Or perhaps those are two different levels of explanation of the same phenomenon?
Another kind of 'understand' is in play when we grasp how the delusion emerges out of prodromal disturbances of selfhood - of the sort that Louis Sass and Josef Parnas and Giovanni Stanghellini - i.e. phenomenologically minded psychiatrists and psychologists - are so good at characterising. Sometimes, as Sass suggests, this emergence may itself thematise aspects of the character of the prodromally psychotic mind - I think that it is following Heidegger (or Merleau-Ponty?) that Sass calls this 'emblematisation' (in the sense of 'emblems of someone's Being' (emblems of their form of Existence / being-in-the-world)).
And then we have yet another kind of 'understand' that comes into play when we grasp the 'symbolic content' of the delusion - in the way that psychoanalysts shed light for us on psychotic aspects of mental function. This, I suspect most important, sense of 'understand' concerns us with a particular mixture of causal and motivational factors that also reference a different ('autistic' or 'psychotic') mode of functioning (i.e. 'formal cause') of part of the self (nb when psychoanalysts talk about 'mind' they refer, it seems to me, to what most psychologists instead refer to by 'self': thus ego, id, superego are parts of the self, not of the information processing mind). Understanding delusions in this sense of 'understand' involves us in 'reading' them according to a certain pattern: as ways the patient is trying to meet their own libidinal / object-relational needs through a kind of surrogate and narcissistic/omnipotent formation. Coming together here, then, are factors about i) a particular (omnipotent/autistic) mode of function, ii) motivational dynamics and the defence mechanisms (projective identification etc.) used in attempts to meet, deny or destroy the emotional needs, and iii) association-governed 'symbolic' connections to provide a substitute content.
There are undoubtedly other forms of understanding too that can be brought to bear on the question of the formation or of the being of delusion. Many of these will be hybrid in character. For example, if I am to grasp the significance of a delusion of world catastrophe, I will first need to understand how aspects of the subject's selfhood and self-experience have become detached from the ('external') world, resulting in an 'autistic' or 'primary narcissistic' or 'omnipotent' mode of being. And then I will need to think of how what has now become the world (i.e. this subjectivised version of worldhood which is all that the schizophrenic person, in the delusional aspects of their being - i.e. in the ambit of their core psychotogenic complexes - has left to them) is experienced as ending. Thus we can say that the delusion of the ending of the world is the emblematisation of the destruction of the self, perhaps projected safely into the future (think of Winnicott on what one fears as already having happened).
But, yes, my main point here is that many of the forms of understanding we bring to bear on delusion are precisely not compatible with the form we started with - with, that is, the kind of understanding which, if I am right, Jaspers is referring to as being impossible with schizophrenic delusion. I cannot grasp a delusion as the product of to-me intelligible-to-hold beliefs and desires (Davidson's so-called 'primary reasons') or in terms of the ends or the facts or purported facts (which are what we normally mean by actual talk of 'reasons') proffered by the delusional person. If I could, then, (the standard interpretation of) Jaspers' story goes, we simply would not here have to do with delusion in any case. But also in the very bringing to bear of some of these other forms of understanding on the delusional beliefs we are, I want to suggest, already consigning them to the category of the un-understandable (in the ordinary, initial, sense). To offer, say, a defensive-motivational understanding of someone's believing what they do is to assert that an ordinary reason-giving explanation will not cut it here. As to which forms of understanding are and are not compatible with maintaining the propriety of the initial everyday-reason-giving form, well, try it out yourself using different examples. Even at the neurological level, whilst it is clear that ordinary believing will necessarily be subtended by any number of diverse neurological processes, if we are looking at accounts of neurological dysfunction, given that such function or dysfunction must itself surely be defined relative to (say) the reality contact or otherwise of the patient, the possibility even of such 'merely causal' forms of explanation will itself, it seems to me, impugn the sane status of the person in their believing what they do when they cleave to what are to be identified as delusions.
And, just in case its not obvious, why am I going on about this here? Well, psychologists are apt to say something like 'oh that terrible Jaspers, he said we couldn't understand delusions, make them intelligible, but this was to give up on the psychotic person and to fail to reach out to them as a sense-making subject'. My point would be that the ways in which we can and do understand delusion in fact precisely also implicate the delusional subject in a form of senselessness, in a failure of everyday meaning. Far from bringing a greater degree of interpretative charity to bear on the psychotic subject, the psychologist just described succeeds merely in muddying the psychopathological waters - as well as in missing the depth of psychotic damage and, one suspects, the terror attendant on it.