The human brain is amazing and can contain as well as the small bit that thinks it's you, various other 'persons', inclduing models of other people, and even the ultimate wisdom of all humanity: what freud called the superego, or we might call conscience (i can't think how to spell that atm), or we might call 'god within us'. When it feels like it, our 'subconscious' can take on the role of god for us (it's where we got the idea from after all), and pretty convincingly so (or satan, or the NWO, VALIS, ECC etc).
In modern times, when this happens intensely it seems more likely it'll be pathological, but sometimes (more in older days) it can happen non-pathologically and spit out some pretty cool wisdom - most old cultures have a defined role for people who might start 'hearing voices', so these people have a place and their 'psychosis' is allowed to develop within a (relatively) stable and healthy framework (not just 'shamanic' societies - most cultures have a place for mystics, religious wildmen etc, including medieval christians).
In the west where there isn't such a role, too often these 'special' people become the 'mental' people who noone understands. There's a million or so years of evolution behind us which had found a use for these mental types - if there was no useful role for these types of people in a society they wouldn't still be here - our job now should be to find/develop/remember a similar use for them now (eg we could bring back shamanism

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I have experienced some mediumship which i couldn't put down to cold reading. I know that some mediums do what they do completely genuinely - they certainly don't think they're doing a derren brown. Whether the information they sometimes get came from just really good intuition (or subconscious cold reading), telepathy, or actual communication with an afterlife i don't know. (my hunch would be intuition (ie the power of the brain in action) though at a certain level of power, intuition becomes indistiguishable (to us) from telepathy).