As Sally Haslanger writes, proper answers to your question do need to feed on some empirical, in this case historical data. But I suggest that these data can only yield answers through some philosophical unpacking. The category of mental illness is itself a cultural product, insofar as it depends on the notion that 1) our minds can be ill just as our bodies can; 2) and so that they are instantiated, most probably, by the brain. Note, however, that to restate the concept of mental illness in such terms is not to say that it is merely a cultural product, that truths are relative to cultures, that if some people believe our mind is instantiated by our feet then so be it. I say this because the kernel of your question seems to address the matter of whether it is useful to imagine a clear-cut distinction between language use - that is, broadly, culture - and a "substantive" reality. To this I would answer No: what is described by the expression "mental illness" must correspond to a reality which...

The cultural historical moment described by Richard Heck aside, it remains that there was something that Cage was turning on its head when he offered - composed would be the wrong word - the event that is 4'33: the experience of listening to music itself. There would be no history of music if all composers had been like Cage. But there would have been no Cage without music, no content to 4'33 if people didn't know what music normally was. That it is possible to exhibit a urinal in a museum or not play anything in a concert hall and be taken entirely seriously as an artist must be considered a cultural phenomenon, worthy of interest, but not a phenomenon internal to the technical forms that developed over the centuries and that gave us symphonies, songs, paintings and sculptures.

To the question 'how' corresponds some sort of scientific description of the phenomenon - and many scientists are indeed engaged in trying to understand 'how' the human brain has developed, indeed evolved, the sort of consciousness that enables us to ask questions about ourselves in the first place, what you call "self-consciousness". The work being conducted on that front is importand and you might find it illuminating. But there remains the sense that, by asking the question, we are reaching the very edge of our consciousness, beyond which nothing seems very familiar anymore. Science describes the physical, familiar world; it does not address the sense of unfathomability that arises out of our consciousness of what we do not understand. The "explanatory gap" mentioned by David Papineau above is the gap between the answers we can come up with, within our familiar, physical territory, and the persistence of the question beyond the scientific answer. We tend to reduce reality to our answers - for...

The "profound human impulse" mentioned by Richard Heck in his response is worth characterizing further: it is the impulse to believe that there are correlations between dimensions of which we have direct experience (the earthly, the present) and those which lie beyond the realm of experience (the cosmos, the future) . But the believer forgets that these correlations only work in virtue of the other-worldly, cosmic dimensions being and of course remaining other-worldly. Astrologers establish a correlation between the movements of the astrological form (itself a mere collection of unrelated astral bodies connected by lines) and the earthly, biological dimension. But it is only because the form is an intangible construct that it can be meaningfully correlated to life on earth in the first place. By drawing a celestial form one draws a human one. Astrology may thus seem true (but about what?) because the very possibility of attributing meaning to what is intrinsically meaningless confers on it...