This paper connects a stylistic hallmark of Marx's work-a dramatic antipathy to imitation and copying-to his rejection of the epistemology of likeness or "harmony" in French romantic social Utopian thought. A space of the common without social mimesis-not just representation and imitation but competitive appropriation, likeness-based equality, social unity, cultivated resemblance, and so on-is in some ways paradoxical. But Marx upholds a vision of the common as collision, foreshadowing Althusser's notion of aleatory materialism, through a discourse of the atom. He moves from atheistic Epicurean models of abstract individuality and opposition to false universalism, to Hegelian ideas of the disaggregated atoms of political class activity, to a rejection of Buonarroti's ambition to harness self-seeking atoms in the collectivity, to a championing of real rather than ideal collisions. Acutely aware of social mirroring processes in the paradigm of the fetishism of the commodity, Marx puts the Lucretian "uproarious contest" and "hostile tension" of atoms at the core of the nonromantic sociality of the common. © 2010 Association for Economic and Social Analysis.