> Voices and visions have never been commonplace, to my knowledge, but that doesn't necessitate that they are barometers of mental illness.
I'd say at least 20% of the population experiences things like this at at least one point in their life - 18% of Americans say they have seen a ghost, that feels like a good jumping-off point from there. In reality, I expect it would be closer to 30%, because of low reporting rates, and that people who see things but ultimately discredit the experience probably wouldn't answer in the affirmative.
So, I would argue, voices and visions, in a low degree of intensity, are in fact fairly normal occurrences and probably not linked to any unusual mental condition.