One of the most important concepts by Marx is that of Commodity Fetishism:
Whence, then, arises the enigmatical character of the product of labour, so soon as it assumes the form of commodities? Clearly from this form itself. The equality of all sorts of human labour is expressed objectively by their products all being equally values; the measure of the expenditure of labour power by the duration of that expenditure, takes the form of the quantity of value of the products of labour; and finally the mutual relations of the producers, within which the social character of their labour affirms itself, take the form of a social relation between the products.
However, according to the new concept of money of social omniequivalence, “the enigmatical character of the product of labour” does not always arise “so soon as it assumes the form of commodities.” This is because (in social omniequivalence) that enigmatical character does not result from “the form of commodities” itself, but rather from the way we represent money.
There are only two ways we can represent money. Either:
We chose a representation indistinguishable from its represented money.1
We chose a representation inherently distinct from its represented money.2
Commodity Fetishism only arises in the first case: it simply mistakes a monetary representation for its represented money.