In his critique of political economy, Karl Marx aims to reveal the intricacies of capital as a process of social metabolism. His approach—defining key terms in the language of the classical political economists, probing the limits of these definitions, and finally redefining them as formal concepts—relies on the detailed analysis of a particular lexicon that centers around a unifying keyword: value. This paper aims to elucidate one of Marx’s rhetorical techniques—namely, the scientific analogy—in its metabolic, chemical, physical, and biological iterations as Marx attempts to explicate value throughout his seminal work, Capital, Volume I. How do these analogies inform Marx’s derivation of the value form and where does the device of analogy fall short in Marx’s critique? Reading Marx’s recourse to scientific analogy through a rhetorical lens reveals that this tool is pivotal in establishing the study of political economy as an inherently scientific pursuit.