The need of an unconscious is demonstrated by the need of deep sleep and represents the need of biological self-preservation. For an excess of memory would paralyse all possibilities of active life. We would be unable to give to the immediate everyday duties that definite attention which they require. The great number of such memories would utterly destroy all possibility of concentrating on the practical needs. And similarly, an inability to bring the thought-mechanism to rest regularly would end by overwhelming the individual with a myriad of unwanted thoughts and again render the simplest concentration difficult or impossible. The senses do not provide merely the conditions under which we become aware of the external world but also the inhibitory mechanism which prevents us from becoming aware of too much. The range of visual vibrations, for instance, is but a fraction of those which are actually present. Similarly, Nature has ordained that the individual mind should shut out of consciousness more than it is able to attend to, should be a representative mechanism which permits us to concentrate on what is relevant in our personal life without distractions that would render life intolerable.
-- Notebooks Category 19: The Reign of Relativity > Chapter 3 : The States of Consciousness > # 114
