From an article by Theodore Dalrymple:
It is no wonder, then, that Marx speaks only in categories: the bourgeois, the proletarian. For him, individual men are but clones, their identity with vast numbers of others being caused not by the possession of the same genes, but by that of the same relations to the economic system. Why study a man, when you know Men?
Could we substitute Man for Men here? This way:
Why study a man, when you know Man?
I think he meant "humanity as a whole", as opposed to a man (an individual). I wonder why he chose to write Men instead of Man. Maybe it was to avoid the monotony of "man - Man"? I have never before encountered Men meaning "humanity".
P.S. 5 minutes after posting this, I had an idea occuring to me that might explain this use of a plural noun. But now I'll wait for answers.