Why is it that those with active schizophrenic psychosis - in the early 'delusional mood/atmosphere' stage - tend to delusionally experience visible buildings as stage sets - as mere façades?
Thomas Fuchs provides a partial answer in his 2005 paper on delusional mood and delusional perception:
In delusional mood, the environment as perceived by the patient changes into a puzzling, mysterious and stage-like scenery. ... Husserl’s concept of intentional perception ... tells us how perception overcomes its own perspectivity [and] may also be understood as describing the intersubjective constitution of reality through common concepts which structure intentional perception: we see a table because we can name it a table as well. ... With the weakening of intentional perception in schizophrenia, objects and situations are not perceived as such any more, but as mere images or enigmatic sceneries. They lose their common-sense significance and turn into a ubiquitous puzzle. Strictly speaking, the significances may still be known in an abstract way, but they have stopped to mean anything to the patient. He is not able to relate them to a common intersubjective world any more.
Henriksen & Parnas also give something of an answer in their 2019 paper on delusional mood:
In delusional mood, the world appears strangely, yet indefinably different. Events or objects lose their natural sense of familiarity, purposefulness, and coherence, and the world itself becomes increasingly uncanny (“unheimlich,” literally meaning “non-homely”)—that is, the patient no longer feels at home or at ease in the world. ... In delusional mood, the context’s mutually implicative referential functions are somehow loosened or weakened, decontextualizing singular perceptual elements from their ordinary, contextual embeddedness and enabling new, unfamiliar meanings to emerge alongside the familiar ones (cf. Matussek 1987: 90). For example, the casual look of a stranger in the street or the tone of voice of a waitress could indicate that they “know” what is going on; the manufacturer’s print on the coffee mug could be a sign pointing to the nature of the impending, etc. Eventually, the patient may experience that objects, events, or others, as Fuchs puts it, no longer “present” themselves in the phenomenological sense of the term, but “only pretend to be just themselves” (Fuchs 2005: 136; author’s italics), leaving an impression of a strange, unreal, artificial, or staged world—a typical form of derealization in schizophrenia...I'd like to supplement these with the answer given by Morag Coate in her 1964 autobiographical Beyond All Reason. Coate's philosophy of perception is enmeshed in misleading, non-phenomenological, merely empiricist/Cartesian/Kantian assumptions. Nevertheless her lived experience shines through, as do the possibilities of its phenomenological re-theorisation:
We take for granted the knowledge of the spaciousness and distinctness of our surroundings. But at first, all we knew of space was what our limbs could reach. Sense data from our moving limbs, received and co-ordinated in the brain, gave us a three-dimensional map of our immediate environment; to this was added information interpreted with growing skill from a pattern of sense impressions received on the small curved surface of each retina. ...
I have plenty of evidence that the room in which I write this is a real room. My view of it makes it possible for me to touch at will the various objects in it; my friends find it as real as I do myself. The fact remains that the room I see is an image projected in my own inner imagination. The image is based on externally received sense impressions, but an image it remains. ...
The space I see now extends only as far as the surface of the walls of my room, except where the windows give me a view of houses opposite and of sky beyond. This is the limit of the environment that I can derive from my senses; the rest is built up in imagination. I am aware beyond the wallpaper of the thickness of walls, and beyond them of other rooms whose known contents I can visualise. The houses opposite have rooms in them whose contents I do not know, though I can vaguely picture them. Beyond this street there is the wide extent of the city in which I live. Beyond the seen expanse of sky is the earth's atmosphere, and outer space, and a myriad unseen stars.
This imagined, though not imaginary, world is quickly altered or even disintegrated by illness. Suppose I suddenly become psychotic while writing this. I can now look across at the houses opposite and see that they are no more than a façade; beyond them is nothingness, and anyone who goes through their front doors will walk into oblivion. And if I go out of my room and walk downstairs I shall enter a different atmosphere which now envelops the earth's surface to a height of ten feet and has the effect of producing complete loss of memory. If at this point the tenant of the downstairs flat arrives to borrow a shilling for her gas meter, I shall be aware that an unseen spirit has entered into her and protected her from harm, and further that my shilling when put into her downstairs meter will affect the quality of the flame from her gas stove and so start regenerating the atmosphere down below.
I have invented this on the spur of the moment. I do not believe a word of it. That is because the imagined framework of my reality environment, based on past experience of life, is still intact. It is important to remember that we have no factual evidence for the vast proportion of what we know; once we have learnt to interpret the more usual phenomena of life we arrive at conclusions by leaping from one piece of evidence to another, and our mental faculty for doing this with reasonable accuracy enables us to lead a normal life. I have never in fact seen or touched the back walls of the houses opposite, and have never enquired from anyone else whether such walls exist. And if they did, I cannot prove that they have not suddenly disappeared. I simply take it for granted that they are there. I have built them into my mental organisation, and so long as this remains intact they can only be demolished by rational means.
In psychotic illness of the kind I have experienced, sense impressions are received clearly and with unusual vividness, but the mental organisation behind them and the critical faculty which sorts and examines and integrates internal ideas with external sense data is no longer functioning normally.