Impossibilism is a Marxist theory and perspective on the emergence of socialism that stresses the limited value of political, economic, cultural, and social reforms within a capitalist economy. It argues that the pursuit of such reforms is counterproductive because they stabilize and strengthen support for capitalism, thereby helping to ensure its continuation. Impossibilism holds that the pursuit of reforms should not be a major concern for socialists because such reforms are irrelevant, if not counter-productive, to the realization of socialism.
Impossibilism insists that socialists should solely (or at the least, primarily) 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) and 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 such as labor unions and electoral politics, thereby de-emphasizing the ultimate objective of building a socialist economy. As a result, socialists who embraced possibilism and immediatism sounded and acted little different from non-socialist reformers.
Impossibilist movements are also associated with anti-Leninism in their opposition to both vanguardism and democratic centralism.
The concept of impossibilism — though not the specific term — was introduced and heavily influenced by the American Marxist theoretician Daniel De Leon, on the basis of theory that De Leon generated before his interest in syndicalism began (see De Leonism). It came to be focused especially on the question of whether socialists should take part in government and pursue policy reforms that benefited the working-class under capitalism.