May 25, 2018:
"Overdetermination" is Freud's concept for thinking causality complexly. Neurotic symptoms are the results of multiple determinations: they're over-determined. Althusser borrows the concept to identify what he underscores as a fundamental distinction between idealist and materialist dialectics. In Hegel, contradiction is simple: a binary opposition between two poles, of which all other phenomena of the whole containing the contradiction are merely emanations, or expressions, or consequences. In Marxism, concrete contradictions are not binary, but are multiply determined local regions of the global totalities to which they belong.
A simple but I think useful example. Where a left-leaning Hegelian might understand capitalism characterized by a binary antagonism between capitalists and proletarians, Marxists insist that multiple intermediate and secondary social strata are necessary to the analysis. Peasants, landowners, intellectuals, managerial professionals, small business owners, the very large number of unemployed or semi-employed, all play significant roles in political dynamics: for example the role of homeless former soldiers and lumpen proletarians as street troops for the Fascist movements between the wars. These strata overdetermine the main antagonism between the two primary forces. Histories and social phenomena — ideologies, cultures, the state, and so on — add to the complexity. "What can this mean but that the apparently simple contradiction is always overdetermined?" Overdetermination was always present in the work of the classic Marxists without the concept being explicitly produced.