Impossibilism is a Marxist theory that stresses the limited value of political, economic, and social reforms under capitalism. As a doctrine, impossibilism views the pursuit of such reforms as counterproductive to the goal of achieving socialism as they stabilize, and therefore strengthen, support for capitalism. Impossibilism holds that reforms to capitalism are irrelevant or outright counter-productive to the goal of achieving socialism and should not be a major focus of socialist politics.[1]

Impossibilists insist that socialists should primarily or solely focus on structural changes (sometimes termed "revolutionary changes") to society as opposed to advancing social reforms. Impossibilists argue that spontaneous revolutionary action is the only viable method of instituting the structural changes necessary for the construction of socialism; impossibilism is thus held in contrast to reformist socialist parties that aim to rally support for socialism through the implementation of popular social reforms (such as a welfare state). It is also held in contrast to those who believe that socialism can emerge through gradual economic reforms implemented by an elected social democratic political party.

Impossibilism is the opposite of "possibilism" and "immediatism". Possibilism and immediatism are based on a gradualist path to socialism and a desire on the part of socialists to help ameliorate the social ills immediately through practical programs implemented by existing institutions including labor unions and electoral politics, thereby de-emphasizing the ultimate objective of building a socialist economy. This position is justified by the fact that socialists who embraced possibilism sounded and acted little different from non-socialist reformers in practice.[2]

Impossibilist movements are also associated with anti-Leninism in their opposition to both vanguardism and democratic centralism.