Beats me how Foucault can be defended as a Marxist, or even as a historical materialist. I'm no Foucault expert, and all I've snorted through is The Order of Things (which should actually be called Words and Things*), but jeezus, if a love affair with Nietzsche isn't enough, his central claim gives him away: "in a culture, at a given time, there is never more than one épistémè that defines the conditions on the possibility of established knowledge". Science governed by social paradigm, what? That would make Foucault's Marx closer to Freud than to Hegel.
At ground level -- which is where I insist on sitting -- dialectic itself, surely, assumes there's something beyond thesis and antithesis toward which the historical dialogue is tending, and this, if I'm not mistaken, is the position taken by the humanist Marx. (That something doesn't have to be Truth, btw. We don't have to posit Truth in order to claim that, through dialogue (interaction), ideas and societies can change, develop, mature.) I think, in fact, that Foucault's Marx is a chimera, that there exists no Marx "who calls into question all production of meaning as rooted in the invisible play of forces" ("le Marx qui met en cause toute production de sens comme enracinée dans le jeu inaperçu des forces" Bernard Sichère, http://www.france.diplomatie.fr/culture/france/biblio/folio/philo2/philo01.html). Marx maintained that we think within and according to our circumstances. That doesn't imply that, in the process, we forget what went before.
If you claim to take a historical view of epistemology and then follow that up by saying, but actually each knowledge-set is valid only for itself, then you're saying there is no dialectic, because norm subsumes scientific method in the pursuit of understanding (let's say, so as not to say truth), and the word is the thing. The text is the thing. But if texts are the prime movers, and not the interplay of texts, then history's irrelevant. Seems to me. Can't have your cake and eat it, eh? Or have I got that wrong.
My old prof, Vincent Descombes, maintained that Foucauldian thought was really an amalgam of four phases: (1) thoughts on the systems of thought; (2) 'genealogy' of disciplinary societies (prison, clinic, asylum); (3) cartography of the 'practices of subjectification', emphasising sexuality and governmental rationality; and (4) philosophy derived from Nietzsche and Heidegger. He also said somewhere that US reception has appeared to concentrate on Foucaults 2 and 3, probably because that reception has occurred primarily in the domains of literary and cultural studies; whereas the French think of him as a neoNietszchean. If that's true, it might help explain the discrepancy in thinking, here on LBO.
cheers, Joanna S
*Les Mots et les Choses
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