"The "means of labor" are one of three basic factors of the production process (Marx, 1967, p 174), along with "human labor", and the "subject of labor" (the material worked on).In some formulations, the "means of labor" and human labor (including the activity itself, as well as the skills and knowledge brought to the production process) comprise the productive forces of society (e.g., Sheptulin, 1978), other formulations define productive forces more narrowly as the union of "instruments of production" and the workers who wield them (e.g., Institute of Economics, 1957).