Insanity

Insanity is always due to a vital attack, or rather possession although there is often a physical reason as well. Hysteria is due to a pressure from the vital world and there may be momentary possessions also. The same thing cannot be said of ordinary delirium the cause of which is physical only—except in so far as all illness is an attack of lower forces of Nature, but these lower forces are not vital beings or what we call specifically hostile forces—they are simply performing their role in nature and of course there may be and probably is a being of some kind presiding over each kind of illness—in Bengal they give a special name to some of them and worship them as goddesses to avert the visitation. But as I say these are really Forces, not vital hostiles.

As for the interest of vital beings in possessing men—beings of the vital world are not constituted like men—they take a delight in struggle and suffering and disorder—it is their natural atmosphere. They want besides to get the taste of the physical world without being under the obligation of taking on birth and developing the psychic being and evolving towards the Divine. They wish to remain what they are and yet amuse themselves with the physical world and physical body.

Ref : Letters on Yoga Vol. III