Marxist and socialist theory developed side by side with classic sociology, but only partly in confrontation with it, as Marxist theorists saw their theoretical contributions primarily as part of their political struggle. The concept of brotherhood or fraternity in the French revolution was made into a key concept in the bourgeois revolutions in Europe in 1848, but the defeat of bourgeois democrats meant a setback for the concept of brotherhood as well. However, in the first decades of the nineteenth century, the idea of brotherhood between workers started to spread. The concept of brotherhood in the first labour organisations referred to a proletarian mental attitude that should stimulate class-consciousness and the insight that workers had common interests (Brunner et al. 1972). In Germany, Ferdinand Lassalle (1825–64) made solidarity a theme in his writings in the 1850s. He distinguished between corporatist and human solidarity. Corporatist solidarity is developed in the sphere of labour, but is too restricted and should be universalised into a general human solidarity, Lassalle maintained (Zoll 2000).

Marx developed his theories and conceptual language as an integrated part of the labour movement struggle that was in the process of developing. The essential innovation of this language was its instrumentality in uniting the working class and constituting it as a subject in the struggle against a defined adversary –the bourgeoisie. The new way of understanding the idea of solidarity was a part of this project.