The Belgian physicist Lééon Rosenfeld (1904––1974) has been called "the clarifier" of Niels Bohr's ideas in quantum mechanics. He had a unique connection to Bohr as his close collaborator from 1930 until Bohr died in 1962. However, although he is considered Bohr's spokesman, he had his own characteristic way of articulating Bohr's views. In particular, Rosenfeld added a dialectical-materialist layer to Bohr's ideas in certain contexts. Despite Rosenfeld's open profession of Marxism, the literature has generally avoided discussing its relevance for his work and activities. This paper suggests some reasons why and investigates Rosenfeld's combination of Marxism and complementarity, which his colleague and friend Wolfgang Pauli metaphorically denoted by the equation "Rosenfeld = √√Trotsky ×× Bohr." Rosenfeld's particular views appear anomalous to the frequently expressed stereotypes of both Marxism and complementarity. The paper concentrates on the role Marxist philosophy played in Rosenfeld's fierce defense of complementarity from the late 1940s through the 1950s and the reception his campaign was accorded at the Niels Bohr Institute in Copenhagen and elsewhere.

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