"It will suffice to know very schematically that an ideology is a system (with its own logic and rigour) of representations (images, myths, ideas or concepts, depending on the case) endowed with a historical existence and role within a given society. Without embarking on the problem of the relations between a science and its (ideological) past, we can say that ideology, as a system of representations, is distinguished from science in that in it the practico-social function is more important than the theoretical function (function as knowledge). [...]
"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. For example, this utopia is the principle behind the idea that ethics, which is in its essence ideology, could be replaced by science or become scientific through and through; or that religion could be destroyed by science which would in some way take its place; that art could merge with knowledge or become 'everyday life', etc. [...]
"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 recognition of its necessity enable us to act on ideology and transform ideology into an instrument of deliberate action on history.
"It is customary to suggest that ideology belongs to the region of 'consciousness'. We must not be misled by this appellation which is still contaminated by the idealist problematic that preceded Marx. In truth, ideology has very little to do with 'consciousness', even supposing this term to have an unambiguous meaning. It is profoundly unconscious, even when it presents itself in a reflected form (as in pre-Marxist 'philosophy'). Ideology is indeed a system of representations, but in the majority of cases these representations have nothing to do with 'consciousness': they are usually images and occasionally concepts, but it is above all as structures that they impose on the vast majority of men via a process that escapes them. Men 'live' their ideologies as the Cartesian 'saw' or did not see - if he was looking at it - the moon two hundred paces away: not at all as a form of consciousness, but as an object of their 'world' - as their 'world' itself. [...]
"So ideology is a matter of the lived relation between men and their world. This relation, that only appears as 'conscious' on condition that it is unconscious, in the same way only seems to be simple on condition that it is complex, that it is not a simple relation but a relation between relations, a second degree relation. In ideology men do indeed express, not the relation between them and their conditions of existence, but the way they live the relation between them and their conditions of existence: this presupposes both a real relation and an 'imaginary', 'lived' relation. Ideology, then, is the expression of the relation between men and their 'world', that is, the (overdetermined) unity of the real relation and the imaginary relation between them and their real conditions of existence. In ideology the real relation is inevitably invested in the imaginary relation, a relation that expresses a will (conservative, conformist, reformist or revolutionary), a hope or a nostalgia, rather than describing a reality.
"It is in this overdetermination of the real by the imaginary and of the imaginary by the real that ideology is active in principle, that it reinforces or modifies the relation between men and their conditions of existence, in the imaginary relation itself. It follows that this action can never be purely instrumental; the men who would use an ideology purely as a means of action, as a tool, find that they have been caught by it, implicated by it, just when they are using it and believe themselves to be absolute masters of it."- Louis Althusser, "Marxism and Humanism"