While attacking Grün's interpretation of Fourier's psychology, Marx claims that no single attribute of real individuals can indicate the whole man: ‘What manner of man can possibly be deduced from the lobe of his own ear, or from such other feature which distinguishes him from the beasts? Such a man is contained in himself, like his own pimple.’ Yet, Marx attempts such a summary of his conception of ‘human nature in general’ when he says ‘The whole character of a species … is contained in the character of its life activity; and free, conscious activity is man's species character.’ In a sense, any of the major categories in which Marx offers his views, given the web of connections between them, could serve this function. Marx chooses man's ‘life activity’, which refers to all activities that distinguish the human species, because he considers it the most favourable vantage point from which to observe man's other relations.

At the core of ‘life activity’ is productive work; for Marx, ‘productive life is the life of the species. It is life engendering life’. And elsewhere he asserts, ‘As individuals express their lives, so they are. What they are, therefore, coincides with their production, both with what they produce and with how they produce.’ We have already explored the human relations involved in ‘what they produce’, where industry was found to be the ‘exoteric revelation of man's powers’, but not yet in ‘how they produce’.