The paper offers a reconstruction of Marx’s explanation of the origin of surplus-value in the production process. Subsequently, the analysis of the fundamental concepts or “categories” of this explanation leads to their classification embodying two pairs of “registers”. Rules which govern the rational use of these categories are derived as well. Thus Marx’s critique of political economy can be interpreted as a critique of systematic “categorial mistakes” due to breaking these rules when using categories in explanations and definitions. In the second part of the paper (intended to be published in the next issue), these rules are applied to three misinterpretations of Marx’s theory by Michael Hauser, Wei Xiaoping and Ľuboš Blaha.