Peculiarities of Ideology

Louis Althusser writes:

[T]he peculiarity of ideology is that it is endowed with a structure and a functioning such as to make it a non-historical reality, i.e. an omni-historical reality, in the sense in which that structure and functioning are immutable, present in the same form throughout what we can call history, in the sense in which the Communist Manifesto defines history as the history of class struggles, i.e. the history of class societies. [Louis Althusser, "On Ideology"]

A "peculiarity of ideologies," in contrast with other kinds of diagrammatic thinking, is their "non-historical" or "omni-historical reality."  What does this mean?  How does it work?

Milton Friedman claims that positive economics is, in one part, a "set of tautologies".

Viewed as a language, theory has no substantive content; it is a set of tautologies. Its function is to serve as a filing system for organizing empirical material and facilitating our understanding of it; and the criteria by which it is to be judged are those appropriate to a filing system. [Milton Friedman, "The Methodology of Positive Economics"]

We can certainly see a bid for a "non-historical" status in the use of tautologies; however, in its second part, positive economics consists of "substantive hypotheses".

Viewed as a body of substantive hypotheses, theory is to be judged by its predictive power for the class of phenomena which it is intended to "explain." Only factual evidence can show whether it is "right" or "wrong" or, better, tentatively "accepted" as valid or "rejected." [Ibid.]

These should tie the theory back to the particularity of history by means of specific predictions answering, despite being made in a language of tautologies, to experience.

If the predictions prove correct, they are, subject to certain other considerations, accepted as part of the theory.  But what happens if they do not prove correct?  The theory, as a a tautologous language and particular hypotheses, is deemed inapplicable to this kind of situation.  Friedman uses the example of supply and demand that work well with regard to consumer goods but do not work so well with speculative markets.  Apparently, the theory can continue unabated in those areas where it does work while ignoring those where it does not.  Althusser's example of communism seems similar.  When a hypothesis drawn in terms of the communist theory fails to explain some historical situation, it is not the theory or the hypotheses that is questioned or falsified.  The particular historical situation is simply excluded from the ongoing theory's domain.

A key element of a theory being tested by its predictions is that the theory specify up front where and to what it applies.  The success of its predictions then reflect back on the theory itself, as both a tautologous language and substantive hypotheses.  But an ideology, it would seem, only determines if it applies at all by whether its predictions prove successful or not.  In this way, the theory is never questioned, only its applications, and it, as a theory, persists non-historically impervious to its failures.