This essay explores the range of phenomena that can be explained by application of a suitably broad but contentful concept of ideology. According to this concept, an ideology is an evaluative map of the social world, typically featuring an ingroup-outgroup distinction, at least in the case of political ideologies. This concept allows for ideologies that support the existing order and those that challenge it, including revolutionary ideologies. I refute the claim that the concept of ideology is not needed because “voluntary servitude” can be explained as a failure of collective action. I show that ideologies can inhibit revolutionary action as well as stimulate and guide it. They can inhibit revolutionary action by convincing the oppressed that their predicament is natural or inevitable or that they are not being oppressed. They can stimulate and guide revolution by debunking oppression-supporting ideologies and by supplying moral motivation that avoids the calculation of costs and benefits that cause failures of collective action.

This essay provides a comprehensive and critical introduction to ideology. It traces the origins and development of various conceptions of ideology, articulating both what they have in common and their differences. Among the distinctions that we develop are the contrasts between pejorative and nonpejorative conceptions, functionalist and causal conceptions, and conceptions that limit ideologies to supporting existing oppressive orders and those that allow for ideologies that challenge such orders. We also explain the role that ideologies can play in either preventing or facilitating social movements to overturn or improve existing institutions. The concluding section of this essay provides a list of important topics for future research on ideology and emphasizes that, in each case, an interdisciplinary approach is needed to understand them fully.