The work of Marx, who claimed “I am not a Marxist”, has often been misrepresented as an abstract philosophical effort, or worse, an ideology. The importance of the questionnaire throughout Marx’s political life, from its centrality within the IWMA to the survey of La Revue Socialiste, demonstrates the opposite, and underlines the practical import at the core of Marxism: a tool for understanding and advancing proletarian power in the class-struggle.