My sense of Marx's "ascending to the level of the concrete," in addition
to what others have said, is that he is playing with some of the
traditions of philosophy. While most philosophers before him (Plato, to a
certain extent Aristotle, Hegel) privileged knowledge at the level of the
abstract (i.e. considered universal knowledge to be a higher level of
knowledge and the object of philosophy), he would "turn this on its head"
(as he says of Hegel in "The German Ideology") and look to the concrete,
to material life (the site where humans produce their own nature by
"dominating" nature i.e. labor), to find where knowledge, ideas,
consciousness and so forth are generated.
Robert Ausch