The main idea is that there are two levels of the mind, one unconscious and the other conscious, and that the first is much more important than the second in determining what we do. It must be said immediately that Brooks has a terminological problem here. He describes the contents of the unconscious mind as “emotions, intuitions, biases, longings, genetic predispositions, character traits and social norms,” and later he includes “sensations, perceptions, drives and needs.” A majority of the things on this list are “conscious,” in the usual sense of the word, since they are parts of conscious experience. The sense in which they are unconscious, which is what Brooks has in mind, is that they are not under direct conscious control. I may consciously choose from a menu, but I do not consciously choose what foods to like.
It is obvious, without the need for scientific research, that vastly more of the work of the human mind is unconscious or automatic in this sense than conscious and deliberate. We do not consciously construct a visual image from sensory input or consciously choose the word order and produce the muscle movements to utter a sentence, any more than we consciously digest our food. The huge submerged bulk of the mental iceberg, with its stores of memory and acquired skills that have become automatic, like language, driving and etiquette, supplies people with the raw materials on which they can exercise their reason and decide what to think and what to do.