As you probably know, the proponents of ID (intelligent design), Johnson, Behe, Dembski, Calvert, et al, directly challenge that presumption, contending that mainstream science is illegitmately biased in favor of naturalist and materialist explanations of phenomena. In other words they charge mainstream scientists with dismissing out of hand, possible supernatural explanations for such things as the origins of life and consciousness. My friend, Tom Clark, has articulated a response to the ID folk at: http://www.naturalism.org/science.htm#whysceince
As he puts it:
"Monistic naturalism is, therefore, simply the result of sticking with science as one's preferred route to knowledge about the ultimate constituents of the world. God, traditionally conceived of as a non-physical, spiritual being set apart or above nature in some respect, is logically barred from being incorporated into a scientific understanding of the world. Science as it's practiced can't get us to God, since God is exactly that which escapes being pinned down as one of many facts or entities within a unified understanding of existence. Put another way, if science as its currently practiced were successful in proving the existence of God, that god could no longer have the supernatural characteristics traditionally attributed to it. In its pursuit of comprehensive explanations, science tends toward ontological unification, not dualism."
^^^^ CB: Yes, and on the other hand, the old "blind faith" version of faith ( as in the story of Job) requires that belief _not_ be based on observation of evidence, not on experience. So, from that standpoint faith in God based on scientific evidence is not valid.
^^^^^^
As far as historical materialism is concerned. I tend to think of it as a theory, or perhaps better still, a meta-theory of history, and so, is something more than just a philosophy of history. In that sense, I would disagree with the positions of the later Engels and of Plekhanov who contended that historical materialism represented the application of dialectical materialism to history. It seems to me that the materialist conception of history is something that can stand on its own, scientifically, whether or not one embraces diamat. In terms of epistemological or ontological assumptions, I think that all it requires is the type of methodological naturalism which science does not presume but which it inevitably leads to.
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I think Marx, Engels and diamat is Farmelant and Brown's favorite "tennis match". I don't know if we should regale LBO-talk with this classic Marxist-Thaxis debate :>)
As far as the naturalist core of historical materialism, recently I have focussed on Marx and Engels' attention to "necessity" as the key locus of the beginning of scientific inquiry. So, that would be my definition of the beginning of naturalism: uncovering first the necessary conditions of a subject, in this case human history.
This "necessity" is the same one famously discussed by M and E with respect to "freedom".