What is a “mental nightmare”?
When there is a chaos in the brain or a local fever, a particular turmoil in the brain, an overstrain, or if there is a want of control, you let yourself be possessed by mental formations, this is what happens most often—mental formations which, most often, you yourself have made, besides. And as the control of the rational, waking consciousness has gone, all this begins to dance a saraband in the head, with a kind of raging madness; ideas get entangled, collide, fight, it is truly hallucinating. Then, unless you have the power to bring a great peace into your head, a great tranquillity, a very strong and pure light, well, it is ten times worse than a vital nightmare. The worst of a vital nightmare consists generally in fighting with an enemy who wants to kill you, and you strike him terrible blows, and the blows never hit; you exert all your force, all your energy, and you do not succeed in touching your adversary. He is there in front of you, he threatens you, he is going to strangle you and you gather all your strength, you try to strike, but nothing touches him. When the struggle is like that, hand to hand, with a being who throws himself upon you, it is particularly painful. That is why you are advised not to go out of the body unless you have the necessary power or the purity. You see, in this kind of nightmare the force you want to use is the “memory” of a physical force; but one may have great physical strength, be a first-class boxer, and yet be completely powerless in the vital world because one does not have the necessary vital power. As for the mental nightmares, that kind of frightful saraband in the head, one has altogether the impression of going mad.
Ref : Questions and Answers 1950-1951