Marxism and Humanism (1969)

Human societies secrete ideology as the very element and atmosphere indispensable to their historical respiration and life. Only an ideological world outlook could have imagined societies without ideology and accepted the utopian idea of a world in which ideology (not just one of its historical forms) would disappear without trace, to be replaced by science. . . .

And I am not going to steer clear of the crucial question: historical materialism cannot conceive that even a communist society could ever do without ideology, be it ethics, art or ‘world outlook’. Obviously it is possible to foresee important modifications in its ideological forms and their relations and even the disappearance of certain existing forms or a shift of their functions to neighbouring forms; it is also possible (on the premise of already acquired experience) to foresee the development of new ideological forms (e.g. the ideologies of ‘the scientific world outlook’ and ‘communist humanism’) but in the present state of Marxist theory strictly conceived, it is not conceivable that communism, a new mode of production implying determinate forces of production and relations of production, could do without a social organization of production, and corresponding ideological forms.

So ideology is not an aberration or a contingent excrescence of History: it is a structure essential to the historical life of societies. Further, only the existence and the recognition of its necessity enable us to act on ideology and transform ideology into an instrument of deliberate action on history.
— Louis Althusser