Freud states explicitly that his concept of the unconscious as he first formulated it was based 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.