"We were speaking just now of the recent hypothesis which attributes sleep to an interruption of the solidarity among the neurons. Even if we do not accept this hypothesis (which is however, confirmed by some curious experiments), we must suppose, in deep sleep, at least a functional break in the relation established in the nervous system between stimulation and motor reaction. So dreams would always be the state of mind of which the attention was not fixed by the sensori-motor equilibrium of the body. And it appears more and more probable that this relaxing of tension in the nervous system is due to the poisoning of its elements by products of their normal activity accumulated in the waking state. Now, in every way, dreams imitate insanity. Not only are all the psychological symptoms of madness found in dreams-to such a degree that the comparison of the two states has become commonplace-but insanity appears also to have its origin in an exhaustion of the brain, which is caused, like normal fatigue, by the accumulation of certain specific poisons in the elements of the nervous system. We know that insanity is often a sequel to infectious diseases, and that moreover, it is possible to reproduce experimentally, by toxic drugs, all the phenomena of madness. Is it not likely, therefore, that the loss of mental equilibrium in the insane is simply the result of a disturbance of the sensori-motor relations established in the organism? This disturbance may be enough to create a sort of psychic vertigo and so cause memory and attention to lose contact with reality. If we read the descriptions given by some mad patients of the beginning of their malady, we find that they often feel a sensation of strangeness, or as they say, of "unreality", as if the things they perceived had for them lost solidity and relief. If our analyses are correct, the concrete feeling that we have of present reality consists, in fact, of our consciousness of the actual movements whereby our organisms is naturally responding to stimulation; so that where the connecting links between sensations and movements are slackened or tangled, the sense of the real grows weaker, or disappears."
Matter and Memory, H. Bergson.15:20 - 16:59