This passage from p. 102 of the introduction was difficult for me, maybe because I’m unfamiliar with Hegel’s Logic. I tried to clarify it for myself with this color-scheme, where red means the less developed concrete whole and orange means simple relations/categories, while blue means the more developed concrete whole and green means complex relations/categories.
“There would still always remain this much, however, namely that the simple categories are the expressions of relations within which the less developed concrete may have already realized itself before having posited the more many–sided connection or relation [i.e. the more complex relation] which is mentally expressed in the more concrete category; while the more developed concrete preserves the same category as a subordinate relation. Money may exist, and did exist historically, before capital existed, before banks existed, before wage labour existed, etc. Thus in this respect it may be said that the simpler category can express the dominant relations of a less developed whole, or those subordinate relations of a more developed whole which already had a historic existence before this whole developed in the direction expressed by a more concrete category.”
Marx makes heavy use of the oppositions abstract vs. concrete, simple vs. complex. When we proceed analytically from the concrete totality, we get to increasingly simple and abstract categories. The totality is concrete and complex, while the relations (and the categories that express them) are abstract and simple. So for example the concrete totality of the capitalist mode of production can be analyzed into simple categories, e.g. money. As a relation, money can be dominant in a less complex totality than capitalism, e.g. in a trading nation like the Roman Empire. In capitalism, a more complex totality, money persists as a subordinate relation. The less complex totality also posits the more concrete categories, e.g. capital, which then determine the development of the more complex totality. Marx clarifies this on p. 103: “Thus although the simpler category [e.g. money] may have existed historically before the more concrete [e.g. capital], it can achieve its full (intensive and extensive) development precisely in a combined form of society [e.g. capitalism], while the more concrete category [capital] was more fully developed in a less developed form of society [e.g. the Roman Empire].”
For me this raises the question: in what way is a complex category like capital “more fully developed” in a less “mature” form of society like the Roman Empire?