See visual hallucination. Visions have been part of human culture throughout the ages, particularly mysticism and religion. In the modern world those who see visions will usually be classed as schizophrenic, visions can also be caused by drug withdrawal or use of hallucinogens. Of course there may be a further type of vision that represents true religious experience and is not pathological, such as a mystic might see. Otherwise of course you could argue that all mystics who see visions are in reality ill. Maybe mystics are using the so-called God spot, but little is yet known about its function, maybe more researach will clarify this issue. See schizophrenia for a little more on the debate.