In this paper I argue for the following view: there was indeed a break in Karl Marx’s work. His work is divided into two periods: the Early and the Later. But the break is not (only) epistemological but (mainly) ontological. This break is the product of deep philosophical reflections related to the hugely important conception that capital is a relation between living and dead labour. To understand this relation, Marx abandoned nominalism, which was the basic philosophical commitment of his youth and embraced — quite reluctantly at first but strongly enough later — realism about universals. This radical rapture occurs between 1847 and 1849 and is found in the series of articles published under the title: Wage Labour and Capital. The key to this transformation is the change of view on the issue of abstraction, which, between 1845 and 1859, turns from an epistemological obstacle into an indispensable tool for scientific knowledge.