June 2, 2018:
"Determination in the Last Instance" is Althusser's non-reductive way of thinking the role that "the economy" plays in the complex articulation of social structures. In Marx's famous base/superstructure metaphor, the elements of the superstructure — the state, law, ideology, and so on — have their own histories which are relatively autonomous. Within the structured totality of a concrete social formation, one element dominates the others: Althusser somewhere suggests that in feudal social formations the dominant element was religion. The element that dominates is determined — in the last instance — by the economy.
"In the last instance": he means indirectly, in the long run, over time. Where a vulgar materialist — Michael Moore, say — will argue that political decisions are determined by direct economic interests — somebody wants a pipeline, there will therefore be a war — Althusser insists that politics, like culture or ideology or the other superstructural elements, has its own history. You can't reduce it to the economy or any other determinism. "From the first moment to the last, the lonely hour of the ‘last instance' never comes."