Epilepsy (often referred to as a seizure disorder) is a chronic neurological condition characterized by recurrent unprovoked seizures. The condition is named from the Greek epilepsia ("a taking hold of or seizing").
In the past, epilepsy has been associated with religious experiences and even demonic possession. Historically, epilepsy was called the "Sacred Disease" because people thought that epileptic seizures were a form of attack by demons, and that the visions epileptics experienced were sent by the Gods. Hippocrates remarked that epilepsy would be considered divine only until it was understood.
There has also been serious speculation that science fiction author Philip K. Dick suffered from similar seizures; he claimed to have experienced visions on several occasions, that, among other things, reportedly helped him save his infant son from an undiagnosed life-threatening medical condition.
More common yet less familiar than the physical manifestations of grand mal epilepsy, temporal lobe epilepsy (TLE) is a response to abnormal electrical activity in the parts of the brain controlling feeling and memory.
In TLE seizures, a patient experiences uncontrollable, intense emotions, sensory hallucinations, and vivid memories. Unlike grand mal epilepsy, the intervals between seizures are often marked by a common pattern of personality changes, typically including compulsive writing or drawing and hyper-religiosity.
Famous people who probably had the disease include Vincent Van Gogh, Soren Kierkegaard, St. Paul and Lewis Carroll. TLE's conjunction of personality and physiology impacts on our concepts of personhood, creativity, and free will.
Caravaggio's "The Conversion of St. Paul," above.