For "Western Marxism" — a term introduced by Maurice Merleau-Ponty in 1955 in his Adventures of the Dialectic (1973) to describe the philosophical tendency stemming from Georg Lukács' History and Class Consciousness (1971; originally published in 1923) — no concept internal to Marxism has been more antithetical to the genuine development of historical materialism than the "dialectics of nature". Commonly attributed to Engels rather than Marx, this concept is often seen as the differentia specifica that beginning in the 1920s separated the official Marxism of the Soviet Union from Western Marxism. Yet, as Lukács, who played the leading role in questioning the concept of the dialectic of nature, was later to admit, Western Marxism's rejection of it struck at the very heart of the classical Marxist ontology — that of Marx no less than Engels.