Feminist theory, in the mid- to late 1970s, looked at gender as a system of signs, or signifiers, assigned to sexually dimorphic bodies, which served to differentiate the social roles and meanings those bodies could have. Feminist theory thus argued that gender was a social construct, something designed and implemented and perpetuated by social organizations and structures, rather than something merely "true," something innate to the ways bodies worked on a biological level. In so doing, feminist theory made two very important contributions. The first is that feminist theory separated the social from the biological, insisting that we see a difference between what is the product of human ideas, hence something mutable and changeable, and what is the product of biology, hence something (relatively) stable and unchangeable. The second contribution is related to the first: by separating the social and the biological, the constructed and the innate, feminist theory insisted that gender was not something "essential" to an individual's identity.