The problem with e.g. Butler is inconsistency and vagueness. Look at how she uses the term "sex" in Gender Trouble. IIRC one reviewer counted several different meanings used interchangably.
Anyway, I don't think the comparison of Butler to Marx is sound. Marx never said "Use-values don't have objective existence, they are produced by discourses". He did say it's a historical deed to discover the usefulness of a thing, but no way would he deny the objective reality of the material object and its properties that ultimately make possible the usefulness and its very discovery. Butler, on the other hand, can be viewed (unfortunately, it's hard to tell exactly, given how quickly she shifts from one meaning of a term to the other) as saying there is no sex, in the sense of a material, biological object, and that it's just a product of discourse and institutions etc., just like gender. What you end up with is a lot of trouble, but not the kind Butler wished for, I think.
And it's like that with pretty much the rest of philosophical postmodernism I've read (and I had to read some to get a degree in philosophy). When you abolish any standards of rationality, evidence etc., anything is possible. I mean, given enough time and typewriters, monkeys would eventually produce Hamlet, but...



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For the sake of devil's advocacy, isn't this the same as 'don't read Marx because gulags'?
I'd echo what others have said about generalising about 'post-modernism' or 'post-structuralism', which are probably looser terms than Marxism (which covers Stalin to Pannekoek and more). As far as I can tell, the 'structuralism' to which post-structuralism refers is mainly Lévi-Strauss, who apparently thought that binary oppositions were a human universal, and suggested they were therefore fixed in human nature (as underlying structures in the mind). So post-structuralism would be all those who critiqued the binary, naturalised categories of structuralism. That's probably all the eclectic bunch of Foucault, Derrida, Lacan, Butler etc would agree on. And it would take them 10,000 words to say 'I agree'.
Yeah a lot of this stuff is difficult. But then, have you tried reading Spinoza or Wittgenstein? Philosophy is hard. Ok, there's the Sokal/Bricmont argument that it's not just difficult, but meaningless. That's almost certainly true of some of it (e.g. the bullshit maths). But then, every movement has its fakers and blaggers. Which would leave the Chomsky argument - if it's not all bullshit, why can't anyone explain it in plain English, since even quantum mechanics can be communicated to a lay audience? Well, sometimes I think it can, so Chomsky's being a bit disengenous. I mean let's take Butler on gender.
Early feminism took the category of sex as a natural given, but argued for women's equality with men (e.g. Mary Wollstonecroft). From the 50s to the 70s, the sex-gender distinction became prominent, with 'sex' refering to the natural, biological and (relatively) unchanging, and 'gender' refering to the socially constructed roles for men and women. Simone de Beauvoir is probably one of the more influential examples, and this is now pretty much the WHO's stance too. Subsequently, feminists started to question the naturalness of 'sex'.
Butler is one of these. Her argument, afaics, basically echoes Marx (though without acknowledging it). Marx points out that it is living in a capitalist society which gives rise to the Robinson Crusoe myth of the political economists. i.e. the particulars of capitalist social relations are projected back onto timeless nature. Living in a capitalist society, going about our business as workers, consumers, political economists etc creates the idea that the laws of this specific society are timeless laws of nature. The idea of what is natural is a product of our activity. Butler does something similar with sex: rather than the gender binary being a product of nature, our understanding of nature is a product of a society with gender binary. So testosterone and estrogen are labelled 'sex hormones' rather than multipurpose growth hormones present in all humans, 'sex' actually refers to a cluster of spectrum variables (chromosomes, gonads, hormones, internal reproductive structures, external genitalia) not a single binary etc.
Any idealism here would be on the part of medical interventions to 'correct' messy natural variations to their 'true' binary form (e.g. castrating babies whose genitals are 'too small', not like a real man, because a woman's just a castrated man etc). Of course, you could make this argument, as I just did, without a lengthy engagement with Freud, Lacan, Foucault, Kristeva, Wittig etc. And you could do it in much plainer language without losing much, and indeed, cutting down on the scope for idealists to misinterpret 'performativity' as voluntarism. But I don't think it's meaningless - just unneccesarily obscure. Though Butler's a literature professor, so kinda goes with the territory. I think Chomsky's test is probably a good one - I just think some post-structuralism passes the test. Foucault probably would, so would Wendy Brown. I dunno about Lacan, Derrida (I've barely read anything on/by them).
I think part of the problem is the 'cultural turn' in theory (towards 'discourse', 'texts' etc) was seen as a rejection of materialism, but the 'Marxist' materialism it was rejecting was often a crude, reductive economism that couldn't theorise culture, gender, sexuality etc except as the superstructure to property relations. I mean in the 70s, homosexuality was often labelled a bourgeois deviation, members of radical communist groups physically attacked women's marches etc. Actually-existing Marxism was really shit on this stuff, which is why post-structuralism's influential imho. I don't really want to defend it as i'm not a big fan myself, and I share a lot of the common criticisms. But it's not just made-up nonsense.
Is it worth doing all the reading around the topic it takes to get your head around, say, Butler? Depends how much you want to engage with it I guess. You could probably get a similar analysis in clearer form via Christine Delphy and Anne Fausto-Sterling. And the problem with reading this stuff is once you start to get your head around the language, it starts slipping into your language too, so you become one of the incomprehensible in-crowd.