I happen to agree with this claim. However it is very often - perhaps most often - pursued in ways that strike me at least as deeply misguided, and which consequently obstruct rather than provide deeper understanding both of the presence and absence of reality-contact and of inner integration.
Here is a typical example. Someone is suffering from a delusional experience - an internal 'passivity' experience of the sort psychopathologists call 'thought insertion' or 'thought withdrawal'. This someone expresses herself by saying 'so and so is taking thoughts out of my mind' or 'dammit I have thoughts in my mind but they are not my thoughts, instead they're Geoffrey's thoughts'.
The philosophical psychopathologist then jumps from this to the claim that, although we've never had to think about or recognise this because we're on the whole sane and so it's all going mentally fine and we don't know any different, actually the normal having of thoughts involves a 'sense of ownership'. It is, he opines, this sense of ownership that the patient suffering thought insertion reveals - reveals in their lack of it that is.
We can then make the account even fancier by distinguishing between senses of ownership and senses of agency etc.: did I conjure the thoughts or merely house them? etc. By this stage the project of philosophical psychopathology will really seem to have got going. My claim is that this is (in a non-clinical sense) but a delusional project.
One way to get this delusional project going is to start by ignoring the strangeness of the psychopathology and taking it that we can simply understand what the patient is saying by gerrymandering some or other metaphysical conception of 'sense of mineness', 'ownership' etc. - whilst disagreeing with her judgement of course as to the origination of her thoughts! (If you want to get this going you'd do best to also falsify the phenomenology of thought insertion, claiming without undertaking any psychopathological research that the positive judgement that the thoughts in question are someone else's is based on an explanatory inference merely from a negative situation of a lack of a sense of mineness etc.) To act thus is, it seems to me, to radically fail the patient, there being little more alienating than someone else's pseudo-assimilation of the deeply strange within oneself.
So to start with, I suggest, we do a lot better to recover our sense of the deep strangeness of the passivity experiences - before we risk explaining it away with some conveniently-concocted-on-the-spot metaphysics of mind.
To find a way into this consider first how what I will (following Rupert Read) call [non-clinical] 'delusions of sense' arise. We have a conceptual metaphor of some sort - perhaps we depict time in spatial terms (backwards and forwards), or depict conversation as a conduit for the transmission of ideas (getting your point across), or we think of mood in positional terms (feeling up and down). But then we start to attribute to what Lakoff & Johnson call the 'target' domain of the metaphor further properties of the 'source' domain which are not contained within the original application. So, for example, we start to move from some basic entitative and spatial metaphors for talk of thoughts and minds ('I've got this thought in my head that I can't get rid of') to the idea of thoughts as objects that can be beamed into people's heads through something called 'telepathy'. Or we get carried away with talk of time in terms of motion and start to imagine that it makes sense to now talk of 'time travel' - as if time were some kind of landscape you can travel through. Or a mathematician might get carried away in thinking of numbers as knowable objects and start to imagine that mathematics is about discovery of hitherto unknown facts regarding the 'super-structure' of the universe.
If someone says 'please excuse me' and leans over and scratches our leg saying 'I have an itch there you know' we know they are being silly - pulling our leg, so to speak, by playing with our language. We know that this silliness is a mere pretend delusion of sense. But we've all met and been people who also get caught up in real delusions of sense - who move from talk of someone being or having a profound or kind soul to talk of their soul as a 'discarnate entity', who start telling themselves that 'time travel' is a real possibility, etc. Usually what has happened is that they've lost track of their conceptual metaphors and started imagining that the target domain enjoys substantive properties of the source domain for which no functional linguistic uses correspond. When we're not just joking about, we merely think we know what we are talking about when we talk in such ways. But in truth we're non-clinically deluded.
Wittgenstein rather thought that much of philosophy was like that: metaphysicians develop delusions of sense which it is the job of the real philosopher to dissolve. We 'take back words to their ordinary uses' - which is to say that we show the metaphysician that he is clearly a) not using the words in question in the ordinary way whilst b) simultaneously failing to provide us with the essential new paradigms for what is actually a new use, which he merely thinks he is entitled to do because c) he has merely pretended to himself and to us that the use to which he is now putting them was all along contained within their old meaning. But enough about Wittgenstein - back to philosophical psychopathology and to thought insertion.
The patient says that she has thoughts in her head that are not her own but instead Geoffrey's. The philosophical psychopathologist of the ilk I am imagining says that our job is to show how this is an intelligible experience. He renders it putatively intelligible by positing various inner abilities which normally - unless we pay attention to what the psychopathology allegedly invites us to consider - we don't even notice as obtaining. But now the psychopathologist has ascribed to the patient what looks like mere mistake and not delusion at all. The patient has a disruption of inner sense, a disturbance in the presentation of their thoughts to themselves - so they mistake them for someone else's thoughts. So in this sense we lose the delusionality of the delusion, rather than make some headway in grasping their thought in its clinical delusionality. And at the same time we make not more sense of ordinary mindedness but rather foist on ourselves a delusion of sense - like the psychotic person we start to get carried away by the conceptual metaphors which structure the individuation and attribution of thought. Now we start to think of thoughts as things, things which are presented to us, things which have quasi-sensory ('phenomenal') properties, things which are 'taken up' by us in particular ways. The theorist's conception of thinking - the conception, that is, of a thinker's relationship to his thoughts - is alienated in a similar way to that lived by the patient.
Read 2003). But a clinician who heeds her call is likely to become nothing but smug, whilst his patient, in the derangement of her true delusionality, will be even more lost to any real grasp of her in the shattering and bewildering severity of her mental disturbance.
How then is it that philosophical pathology can help us better understand the nature of sanity and insanity? Well, consider again Louis Sass's above-mentioned project. He thinks we can grasp schizophrenic autism through seeing it as an embodiment of solipsism. Rupert Read disagrees. Because solipsism is what he calls a 'philosophical position' and because Read, like me, is being what I would call a 'good Wittgensteinian'(...), he urges that it is therefore simply nonsensical. But this leads him to erroneously state (2003 p. 136) that "Perhaps in contrast to Sass, there is then nothing [as regards helping us understand schizophrenia] special about solipsism: it is just one mode of presentation rather than another of the illusion that there can be philosophical positions." However this misses the point that it really is particularly the allure of solipsism for us, and not its (unreal - and in its unreality just as generic as any other bit of philosophical nonsense, as Read says) content, that is what helps us to better understand the schizophrenic. We understand something of the psychology of this allure - how we can come to develop delusions of sense best in certain circumstances (when we are staring, lying utterly still, feeling detached from our own bodies, not caught up in all that loop-y re-entrant causal nexus of body-world praxical sensori-motor interaction) - and we see how the clinical delusions then grow in the soil of someone whose 'maximal grip' or sensori-motor attunement - an attunement which constitutes their certainties-in-action that themselves constitute the bedrock of their knowing - is thus loosened. Thankfully the non-psychotic person entertaining solipsism can just snap out of it and like Hume go back to the billiards game. We are not all so lucky.