June 17, 2018:
I think, through these changes of emphasis Althusser was trying to understand why the French state proved to be more resilient than classical Marxism would have predicted. The state should have fallen under the absolutely massive nationwide crisis of May '68. But, it held. How to understand its survival?
The results of this phase were also explosive, but the explosion was contained in one spot: the theory of the ISAs. This is interesting especially because the book-length context of the ISAs essay was invisible at the time. Despite this "silence", the ISA's essay is today Althusser's best-known and most frequently cited work.
I'll call out some less shiny results: redefinition of the break as process irreducible to the before and after of an event; rigorous, step-by-step analysis of Marx's break with Hegel and then Feuerbach and then Hegel again in his texts of 1843-45; acknowledgement and analysis of Hegelian and Feuerbachian survivals in Capital; criticism and abandonment of capital-T Theory as privileged ground; and subsequent redefinition of philosophy.
In my opinion the most important innovation of this period is his attempt to think ideology as materiality, independent of consciousness. But let me also stress that while I find the equivalence of ideology with ritual fruitful, to me it seems "one-sided". There are daily examples of people using arguments as justifications for utter nonsense. These aren't rituals so much as discourses, to which I find Freud's concepts directly relevant. Bill Gates and Mark Zuckerberg deserve their billions because they took the risk of dropping out of Harvard: a narrative which makes invisible their family networks and the circumstances of privilege which brought them to Harvard in the first place. Makes invisible: represses. Climate change deniers angrily attack the science of global warming, a proxy debate about whether capitalism really does bring the greatest benefit to the greatest number. Proxy: displacement. It seems to me that the spoken discourses of ideology frequently conform to the language of dreams: discourses of the unconscious. Althusser not only removes discourse from centrality, he devalues it altogether, suggesting an awkward duality of discourse versus practice in which discourse is in some sense less real. In consequence he demotes the pedagogical model of activist practice — a good thing. Yet at the same time he closes the possibility of intervention into the ideological narratives sustaining social injustice and the looming catastrophe of runaway greenhouse effect. I believe there's strong reason not to surrender those possibilities.
But to stress: the status of many of these texts is unknown. Most of what I've called attention to was not published by Althusser. Why is that? Did he change his mind? Or, simply lack the time to finish? — as he repeatedly noted about Marx. All through this period his writing is criss-crossed with inconsistencies, self-contradictions, incomplete directions. Reading then as Althusserians: Althusser was trying to think thoughts that he lacked the concepts to think. Like Marx he produced elements, but that's all. I think, that remained the situation to the end of his life.