The union of theory and practice became a current concept as a result of the Marxian doctrine, or, rather, of a certain interpretation of that doctrine. This concept as Marx used it is far from being clear, precisely because of the widespread currency it enjoyed. Its meaning can be made clearer by recalling the connection between the Marxian and the Hegelian sets of concepts and attempting to understand the former in the light of a strict interpretation of some of the sources pertinent to the subject.