on the theory of repression. He postulated a cycle in which ideas are repressed, but remain in the mind, removed from consciousness yet operative, then reappear in consciousness under certain circumstances. The postulate was based upon the investigation of cases of hysteria, which revealed instances of behaviour in patients that could not be explained without reference to ideas or thoughts of which they had no awareness and which analysis revealed were linked to the (real or imagined) repressed sexual scenarios of childhood. In his later re-formulations of the concept of repression in his 1915 paper 'Repression' (Standard Edition XIV) Freud introduced the distinction in the unconscious between primary repression linked to the universal taboo on incest ('innately present originally') and repression ('after expulsion') that was a product of an individual's life history ('acquired in the course of the ego's development') in which something that was at one point conscious is rejected or eliminated from consciousness. In his account of the development and modification of his theory of unconscious mental processes he sets out in his 1915 paper 'The Unconscious' (Standard Edition XIV), Freud identifies the three perspectives he employs: the dynamic, the economic and the topographical. The dynamic perspective concerns firstly the constitution of the unconscious by repression and secondly the process of "censorship" which maintains unwanted, anxiety-inducing thoughts as such. Here Freud is drawing on observations from his earliest clinical work in the treatment of hysteria. In the economic perspective the focus is on the trajectories of the repressed contents ("the vicissitudes of sexual impulses") as they undergo complex transformations in the process of both symptom formation and normal unconscious thought such as dreams and slips of the tongue. These were topics Freud explored in detail in The Interpretation of Dreams and The Psychopathology of Everyday Life. Whereas both these former perspectives focus on the unconscious as it is about to enter consciousness, the topographical perspective represents a shift in which the systemic properties of the unconscious, its characteristic processes, and modes of operation such as Condensation and Displacement, are placed in the foreground. This "first topography" presents a model of psychic structure comprising three systems: The System Ucs – the unconscious: "primary process" mentation governed by the pleasure principle characterised by "exemption from mutual contradiction,... mobility of cathexes, timelessness, and replacement of external by psychical reality." ('The Unconscious' (1915) Standard Edition XIV). The System Pcs – the preconscious