No psychical trouble arises in true initiation, but the premature and sudden flashing-up of the astral world may give rise to insanity. In clairvoyance, man is liberated from his physical body. Hence the dangers that may threaten the mind and brain of one who attempts this kind of training without being absolutely balanced.
Source: Rudolf Steiner – GA 94 – An Esoteric Cosmology – Lecture IX: The Astral World – Paris, 2nd June 1906
Translated by R. Querido