Psychologist Jacques Van Rillaer points out that, "the unconscious was not discovered by Freud. In 1890, when psychoanalysis was still unheard of, William James, in his monumental treatise on psychology (The Principles of Psychology), examined the way Schopenhauer, von Hartmann, Janet, Binet and others had used the term 'unconscious' and 'subconscious'". Historian of psychology Mark Altschule observes that, "It is difficult—or perhaps impossible—to find a nineteenth-century psychologist or psychiatrist who did not recognize unconscious cerebration as not only real but of the highest importance."
Van Rilliaer could have also mentioned that Eduard von Hartmann published a book dedicated to this topic, Philosophy of the Unconscious, in 1869 -- long before anybody else.
Furthermore, 19th century German psychologists, Gustav Fechner and Wilhelm Wundt, had begun to use the term in their experimental psychology, in the context of manifold, jumbled sense data that the mind organizes at an unconscious level before revealing it as a cogent totality in conscious form."
An iceberg is often (though misleadingly) used to provide a visual representation of Freud's
theory that most of the human mind operates unconsciously.
Sigmund Freud and his followers developed an account of the unconscious mind. It plays an important role in psychoanalysis.
Freud divided the mind into the conscious mind (or the ego) and the unconscious mind. The latter was then further divided into the id (or instincts and drive) and the superego (or conscience). In this theory, the unconscious refers to the mental processes of which individuals make themselves unaware. Freud proposed a vertical and hierarchical architecture of human consciousness: the conscious mind, the preconscious, and the unconscious mind—each lying beneath the other. He believed that significant psychic events take place "below the surface" in the unconscious mind, like hidden messages from the unconscious. He interpreted such events as having both symbolic and actual significance.
In psychoanalytic terms, the unconscious does not include all that is not conscious, but rather what is actively repressed from conscious thought or what a person is averse to knowing consciously. Freud viewed the unconscious as a repository for socially unacceptable ideas, wishes or desires, traumatic memories, and painful emotions put out of mind by the mechanism of psychological repression. However, the contents did not necessarily have to be solely negative. In the psychoanalytic view, the unconscious is a force that can only be recognized by its effects—it expresses itself in the symptom. In a sense, this view places the conscious self as an adversary to its unconscious, warring to keep the unconscious hidden. Unconscious thoughts are not directly accessible to ordinary introspection, but are supposed to be capable of being "tapped" and "interpreted" by special methods and techniques such as meditation, free association (a method largely introduced by Freud), dream analysis, and verbal slips (commonly known as a Freudian slip), examined and conducted during psychoanalysis. Seeing as these unconscious thoughts are normally cryptic, psychoanalysts are considered experts in interpreting their messages.
Freud based his concept of the unconscious on a variety of observations. For example, he considered "slips of the tongue" to be related to the unconscious in that they often appeared to show a person's true feelings on a subject. For example, "I decided to take a summer curse". This example shows a slip of the word "course" where the speaker accidentally used the word curse which would show that they have negative feelings about having to do this. Freud noticed that also his patient's dreams expressed important feelings they were unaware of. After these observations, he came to the conclusion that psychological disturbances are largely caused by personal conflicts existing at the unconscious level. His psychoanalytic theory acts to explain personality, motivation and mental disorders by focusing on unconscious determinants of behavior.
Freud later used his notion of the unconscious in order to explain certain kinds of neurotic behavior. The theory of the unconscious was substantially transformed by later psychiatrists, among them Carl Jung and Jacques Lacan.
In his 1932/1933 conferences, Freud "proposes to abandon the notion of the unconscious that ambiguous judge".
Carl Gustav Jung, a Swiss psychiatrist, developed the concept further. He agreed with Freud that the unconscious is a determinant of personality, but he proposed that the unconscious be divided into two layers: the personal unconscious and the collective unconscious. The personal unconscious is a reservoir of material that was once conscious but has been forgotten or suppressed, much like Freud's notion. The collective unconscious, however, is the deepest level of the psyche, containing the accumulation of inherited psychic structures and archetypal experiences. Archetypes are not memories but images with universal meanings that are apparent in the culture's use of symbols. The collective unconscious is therefore said to be inherited and contain material of an entire species rather than of an individual. Every person shares the collective unconscious with the entire human species, as Jung puts it: [the] "whole spiritual heritage of mankind's evolution, born anew in the brain structure of every individual".
In addition to the structure of the unconscious, Jung differed from Freud in that he did not believe that sexuality was at the base of all unconscious thoughts.
Mark Blechner sees the unconscious mindbrain as including all thoughts, emotions, and memories that are not conscious. This can include mental contents that have been actively repressed, but also many other mental properties and reactions that occur without awareness. Unconscious mentation can include tendencies towards prejudice  and biases about the subjective impact of loss and reward These mental operations are universal and shape waking behavior without conscious awareness. Nisbett and Wilson write: “There may be little or no direct introspective access to higher order cognitive processes.” Mental processes including problem-solving can occur unconsciously in waking or in dreams. The conscious mindbrain may have difficulty accessing and comprehending solutions arrived at unconsciously.
Blechner (1983) cites data from patients with borderline personality disorder, whose dreams tend to be less bizarre and more like rational statements while their waking thoughts are more bizarre and chaotic, and concludes: "If we assume that dreams reflect unconscious processes, then the pattern of borderline dream structure may lead us to conclude that it is erroneous to conceptualize unconscious thinking in terms of specific types of content or process as in classical psychoanalytic theory. If we avoid the reified notion of 'the unconscious' as an entity, which is limited to any kinds of structural features or types of contents, and adhere to the usage of 'unconscious' only as an adjective, describing a quality of experience, without awareness, which may apply to any forms of thinking and emotional states, we may begin to consider in some patients that that which is unconscious may well be the kind of rational thinking and variegated affect that form the bulk of conscious experience in less troubled individuals."