May 24, 2018:
So what's a "Science", then? Althusser following Bachelard rejects positivist, empiricist and common-sensical conflation of science with the apparatus of experimentation and other practices which validate scientific theories, as he also rejects naive definitions of science as simple induction/generalization from observation. Science is a type of discourse in which concepts at a high level of rigor interact in a "field" which makes them possible and which they simultaneously define. It's the rigorous systematicity of the conceptual field structured around an appropriate theoretical object. It's characterized by the production of new knowledges, where knowledge is distinguished from mere intuition or whathaveyou as the product of a process of theoretical labor. Althusser sees that as the spiral form of circularity, not as tautology. Science is a system of concepts capable of producing knowledges which make possible new sciences which are systems of concepts which make possible new knowledges. Science is the systematicity of its concepts.
A type of discourse: discourse without a subject. Authors as individualized consciousnesses which "know" things do not exist in scientific discourse. Scientificity depends on this. Scientificity is constituted specifically as supra-personal objectivity independent of the foibles of individual perception or other subjective contingencies. The criterion of reproducibility of scientific experiments is one example: an experiment must be designed to make the individual experimenter disappear. That's part of what makes it scientific.
Science is contrasted with ideology throughout Althusser's work. In this first period, science and ideology are counterposed pretty much as absolutes: independent of each other in their purity, after and before the break. Probably the most important distinction for the earlier Althusser is that ideologies lack objects, the "theoretical objects" which provide the unity of scientific problematics; he drops this later. The latter Althusser sees a permanent intermingling of science and ideology, where sciences are always under siege by their own ideological prehistories.
Indeed, Althusser's project exists because of this. Marx founded the science of history. Yet Marx's science is under ideological siege, not from without, by the anti-Marxist cold warriors one would expect, but from within, by philosophers and party officials who evoke Marx's name and legacy while substituting Marx's ideological prehistory for the mature science Marx founded. In his middle and later periods Althusser says: this always happens. His examples from the histories of concrete sciences illustrate and underscore his demonstration. This is the crux of what many of his concepts are designed to achieve: a definition of science which is itself scientific.