The most creative and radical achievement of Freud’s theory was the founding of a “science of the irrational” – i.e., the theory of the unconscious. As Freud himself observed, this was a continuation of the work of Copernicus and Darwin (I would add also, of Marx): they had attacked the illusions of man about this planet’s place in the cosmos and his own place in nature and in society; Freud attacked the last fortress that had been left untouched – man’s consciousness as the ultimate datum of psychic experience. He showed that most of what we are conscious of is not real and that most of what is real is not in our consciousness. Philosophical idealism and traditional psychology were challenged head-on, and a further step was taken into the knowledge of what is “really real.” (Theoretical physics took another decisive step in this direction by attacking another certainty, that concerning the nature of matter.)
Freud did not simply state the existence of unconscious process in general (others had done that before him), but showed empirically how unconscious processes operate by demonstrating their operation in concrete and observable phenomena: neurotic symptoms, dreams, and the small acts of daily life.
The theory of the unconscious is one of the most decisive steps in our knowledge of man and in our capacity to distinguish appearance from reality in human behavior. As a consequence, it opened up a new dimension of honesty and thereby created a new basis for critical thinking.
Erich Fromm – The Crisis of Psychoanalysis p.5