Mind Hacks shares this excerpt from Hallucinations and Their Impact on Art:
What is more curious to the contemporary man is that the medieval description of insanity does not include hallucinations; and the experience of possession (passivity phenomena) is not described as occurring concurrently with or as part of a visionary state.
In Western Europe from AD 500-1500, people who heard voices or saw visions considered themselves, and were considered buy their contemporaries, to have had an actual perceptual experience of either divine or satanic inspiration. They were not considered mad and were not treated as such. Hallucinations (fantasmata) were only considered mad when combined with trickery (prestigiae).