Ivan Szelenyi continues Yale's Foundations of Modern Social Theory (SOCY 151) with a lecture on Marx's mature turn toward historical materialism. He traces the shift from the young Marx's emphasis on action and change to the later Marx's more deterministic, science-minded account of capitalism's inevitable collapse. Working closely through the Theses on Feuerbach, Szelenyi examines competing theories of truth, asking whether it resides in the subject, the object, or their interaction, and how Marx's materialism grounds truth in sensory experience and activity rather than passive reflection. The lecture moves on to The German Ideology, laying out modes of production, forces and relations of production, and the division of labor. It closes with two central claims: that life determines consciousness, and that the ruling class's interests shape a society's dominant ideas, tying Marx's philosophy directly to a sociology of knowledge.