June 9, 2018:
Althusser was a French Communist philosopher whose major works were published in the mid and late 1960s. It's important to stress his Communist Party membership, which was central to his purpose and to the way his work evolved. His project was political: he opposed the rightward turn of the PCF toward what became Eurocommunism in the 1970s. Because he was a philosopher, not a politician, his "interventions" were in philosophy, rather than, say, forming a faction within the Party. He intervened against the philosophical interpretation of Marx which Party officials used to rationalize their rightward direction, clarifying the relationship of the mature Marx of Capital to the young Marx of the 1844 Manuscripts; analyzing Marx's complicated relationship to Hegel; and defending the claim that Marx inaugurated the science of history, in the same sense that Thales inaugurated the science of mathematics, Galileo and Newton the science of physics, Lavoisier that of chemistry, Darwin and Mendel of biology, and Freud the science of psychology.
To intervene coherently, Althusser developed philosophical tools which make it possible to draw non-subjective, non-arbitrary "lines of demarcation" within any body of thought. He introduced the idea of reading as a noun: a method of interrogating texts based on explicit protocols; and produced original and highly rigorous scientific concepts which make it possible to think scientificity itself. As a consequence he enabled for the first time a non-subjective, non-arbitrary history of ideas, with fruitful implications far beyond his immediate purpose within the Party.
His goal was to produce Marx's philosophy. That is, clarify and systematize the philosophy Marx never had time or tools to write. Looking back on his career, he wrote, "We tried to make the works of Marxism, Marxism itself, and, in the final analysis, the work of Marx himself, readable and thinkable. Which means that, previously, it scarcely was..." It was a labor of both reconstitution and criticism: a reclaiming and a going-beyond. Where produce means: perform the theoretical labor necessary to generate a specific kind of useful product, one which is itself theory.
His interest for non-activists is in: 1) the technical practice of reading which he produced; 2) the theoretically rigorous history of ideas he enables; 3) the scientific definition of science he developed; 4) his analyses of the histories of concrete sciences and their transformations; and 5) his hyper-rigorous readings in the history of philosophy. His interest for Marxists is in: 1) his systematic re-conceptualization of ideas which Marx produced without in every instance producing their concepts in rigorous form; 2) his exploration of the "limits" of Marx's thought, which also means its limitations; and 3) his attempts to overcome the most urgent of these. His interest for activists is in the usefulness of the tools he produced. You can utilize Althusser's analytical tools to decide whether your coalition against police violence should ally with the Democratic Party, or not. I don't think it would make sense to use Derrida's concepts in the same way.
Althusser's work falls reasonably into three periods defined by his own emphases and conclusions; the status of his philosophical tools as he refined them; the philosophical conjuncture in France; and the political conjuncture in France. I'll sketch them briefly, labeling them, 1) Theoreticism; 2) Ideology; and 3) Marx in His Limits. I'll outline the key concepts he produced, or borrowed and transformed, as they evolved in each period; and try to quickly note something of the results. This primer isn't intended to be comprehensive: just an outline within which you can situate particular texts. You should be able to pick up any of his works in English and understand its context. In closing I'll suggest some practical questions which Althusser's tools are well-adapted to answer.