Being conscious is one of the traditional criteria for having an hallucinations (since while unconscious they're dreams). If you're looking for a particular mechanism various drugs can produce them (even marijuana for a sufficiently clinical definition of hallucinations) as can sensory deprivation and lack of sleep.
Graffiti outside Latin class.
Sed quis custodiet ipsos custodes?
A juvenal prank.
I think he means just doing it through thought. No drugs. No sleep deprivation. Just sitting at the table one day and deciding I wanna see crazy stuff.
The brain is such a riddle that there is no answer for that one.
Read some Carlos Castaneda or something. Though they used lots of drugs.
I once slept for under 1/2 hour in a week a few times. I couldn't tell if I was in a dream or reality anymore.