One of Bion's most unique contributions to psychoanalysis is his conception of dreaming in which he elaborates, modifies, and extends Freud 's ideas. While Freud dealt extensively with dream-work, he showed more interest in dreams themselves and their latent meaning and theorized that dreams ultimately constituted wish-fulfillments originating from the activity of the pleasure principle. Bion, on the other hand, focuses more on the process of dreaming itself and believes that dreaming occurs throughout the day as well as the night and serves the reality principle as well as the pleasure principle. In order for wakeful consciousness to occur, dreaming must absorb (contain) the day residue, and transfer it to System Ucs. from System Cs. for it to be processed (transformed) and then returned to System Cs. through the selectively-permeable contact-barrier. Dreaming, consequently, allows the subject to remain awake by day and asleep by night by its processing of the day's residue. Bion seems to conceive of dreaming as an ever-present invisible filter that overlays much of our mental life, including perception, as well as attention itself. He further believes that dreaming is a form of thinking that normally involves the collaborative yet oppositional (not conflictual) activity of the reality and pleasure principles as well as the primary and secondary processes. He also conflates Freud 's primary and secondary processes into a single 'binary-oppositional' structure (Lévi-Strauss, 1958, 1970) that he terms 'alpha-function', which constitutes a virtual model that corresponds to the in-vivo activity of dreaming. He further believes that the analyst dreams as he or she listens and interprets and that the analysand likewise dreams while he or she freely associates.