Ostensibly, we are discussing money. Marx summarises where we are in that discussion.

The product becomes a commodity. The commodity becomes exchange value. The exchange value of the commodity acquires an existence of its own alongside the commodity; i.e. the commodity in the form in which (1) it is exchangeable with all other commodities, (2) it has hence become a commodity in general, and its natural specificity is extinguished, and (3) the measure of its exchangeability (i.e. the given relation within which it is equivalent to other commodities) has been determined—this commodity is the commodity as money, and, to be precise, not as money in general, but as a certain definite sum of money, for, in order to represent exchange value in all its variety, money has to be countable, quantitatively divisible.

More (pdf, 269 KB): The Chapter on Money Part 5: Money as Commodity (pp. 165-186)