Dealing with the three Althusserian bugbears:
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he mainly wanted to say that society was not fully determined by the economy (except 'in the last instance', whatever that means).
^^^^^ CB: Here's a proposal on what it means. "In the last instance" is a time of fundamental or revolutionary change. Marx and Engels notion is that history or the revolutionary changes in history or the transitions in the mode of production in history since the rise of class divided society are determined or "caused" by contradictions between exploiting and exploited classes. My position is that Marx and Engels were not in this formulation vulgar materialists (smile).
Most of the time of history, of course, is not spent in fundamental transitions between modes of production. Most times are not revolutionary , but conventional. Most times of history are not "the last instance". "The last instances" are rare. Through most of the time of history there is more of a reciprocal determination between superstructure and base, " infra-structure". Also, a revolution is a fundamental change in a societies' structures. In revolutionary eras, the change in the base structure causes a fundamental change in the super_structure_, the culture, the political structure, and especially the property laws, which are structures (part of superstructure)
Specifically, a structure in the Althusserian/Levi-Straussian sense, is a complex idea, built up of algebraic relations between binary oppositions. It is an idea that guides peoples conduct, activity. It's a principle of conduct, a custom, a "value". Often in modern society it is a custom enforced by the state, i.e. a law. One of Althusser's points would be that the "economy" or class struggle is "structured" , too. It is not just the superstructure that has a guiding ideational structure. The class struggle is structured by many rules like the polity is.
Notice the word/root "structure" is in Marx's famous forumulation "superstructure". Levi-Strauss' structuralism , the source of Althusser's structuralism, is a superstructuralism, in a way. Also notice that Levi-Strauss's structuralism derives from anthropology or the study of pre-written historic, or pre-class divided societies in which super-structure is long term determining because there weren't any classes in conflict or revolutions in the mode of production. The modes of production of "primitive" society haven't changed structure much for hundreds of thousands of years. Plus ca change; plus la meme chose. So, Levi-Strauss choice of a metaphor of rigidity ("structure") is in ways more apt in his direct subject matter , the unchanging "primitive" societies, than modern societies. On this Levi-Strauss distinguishes between "cold" ( unchanging, "primitive") and "hot" ( more frequently changing, modern) societies. So, Althusser may be stretching it a bit in applying the Levi-Straussian concept to "hot" France.
Since Marxism does seek to consciously "stir up" a "last instance" for a transition from capitalism to socialism, it focuses on contradictions in the "economy" or more precisely between the ruling class and ruled classes , and from the standpoint of activating the ruled class to act in its material self-interests.
So, yes, most of the time of history society is not determined in the "last instance"; the last instance ( when revolutions occur) is not most of the instances of history (trivially). On the other hand, Marxism is all about making our lifetime a "last instance" in which the working class of the "base" changes the superstructure. Notice in this the working class is a Subject of history. It has "agency" , in the post mod sense. It's not a non-human "structure" or utterly "objectively" determined. Of course, it has to have class and socialist consciousness greater than that of the ruling class' class consciousness to succeed at that historic mission, blah , blah, blah (smile).
In part, this post is an attempt to logically consistently interpret the famous passage from Marx below.
^^^^^^^^
In the social production of their existence, men inevitably enter into
definite relations, which are independent of their will, namely
relations of production appropriate to a given stage in the
development of their material forces of production. The totality of
these relations of production constitutes the economic structure of
society, the real foundation, on which arises a legal and political
superstructure and to which correspond definite forms of social
consciousness. The mode of production of material life conditions the
general process of social, political and intellectual life. It is not
the consciousness of men that determines their existence, but their
social existence that determines their consciousness. At a certain
stage of development, the material productive forces of society come
into conflict with the existing relations of production or – this
merely expresses the same thing in legal terms – with the property
relations within the framework of which they have operated hitherto.
>From forms of development of the productive forces these relations
turn into their fetters. Then begins an era of social revolution. The
changes in the economic foundation lead sooner or later to the
transformation of the whole immense superstructure.
In studying such transformations it is always necessary to distinguish between the material transformation of the economic conditions of production, which can be determined with the precision of natural science, and the legal, political, religious, artistic or philosophic – in short, ideological forms in which men become conscious of this conflict and fight it out. Just as one does not judge an individual by what he thinks about himself, so one cannot judge such a period of transformation by its consciousness, but, on the contrary, this consciousness must be explained from the contradictions of material life, from the conflict existing between the social forces of production and the relations of production. No social order is ever destroyed before all the productive forces for which it is sufficient have been developed, and new superior relations of production never replace older ones before the material conditions for their existence have matured within the framework of the old society.
http://www.marxists.org/archive/marx/works/1859/critique-pol-economy/preface.htm