Sven-Eric Liedman’s The Game of Contradictions (2022) sets out to be both the most comprehensive study of Engels’s work on the social implications of 19th-century developments in the natural sciences and the most detailed analysis of this aspect of the Marx–Engels relationship. While noting the richness and depth of Liedman’s research, this essay argues that his critique of Engels as a system builder is one-sided. Liedman’s focus on the negative aspects of system building obscures the positive implications of Engels’s research. Far from damning 20th-century Marxism, Engels’s historical naturalism confirms that his ideas are more relevant than ever. The contemporary environmental crisis is simultaneously a natural and a capitalist crisis, and any adequate attempt to conceptualize it would be well advised to begin from insights both of his and Marx’s works. Unfortunately, despite highlighting overlaps between Marx’s and Engels’s approaches to the relation between social and natural sciences, Liedman’s dismissive approach to Engels’s Dialectics of Nature blinds him to the continuing relevance of this fundamental resource.