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human
When the logical positivists said that they rejected metaphysics, what does that even mean?

I don’t understand how anyone can actually “reject” metaphysics, since the foundation of any worldview is a metaphysical stance, and it seems to me that the underlying metaphysics of positivism is a radical, absolutist materialism. Were they just using metaphysics in a different way, to refer to idealist or dualist metaphysics?
gpt
> Were they just using metaphysics in a different way, to refer to idealist or dualist metaphysics?

Well, yes, as Carnap himself said explicitly at a number of points. The metaphysics that they were concerned with was not simply "questions about what exists" but the kind of a priori speculative reasoning about the world that claimed to be able to show that scientific theories like GR or QM *had* to be false. Note that this wasn't an abstract worry: one of the dominant philosophical schools of thought in Germany in the 10s and 20s held that relativity couldn't be true on the grounds that it violated Kantian principles.

It's worth emphasizing the general point here, but with "epistemology" and "metaphysics" and many other philosophical concepts, one shouldn't assume that the authors before (oh, say) 1970 have exactly what we have in mind today. 

> and it seems to me that the underlying metaphysics of positivism is a radical, absolutist materialism.

That would be shocking to both the positivists and their critics. 

If anything, their view is normally taken to be a kind of naive phenomenalism --- certainly that's the standard misreading of Carnap's *Aufbau* --- in which everything is reduced to sense perception. 

What they're really on about, though, is a kind of in-practice pluralism --- exemplified by Carnap's principle of tolerance --- according to which metaphysical speculation simply isn't relevant *for the purposes of science*. It doesn't matter (for science) what kind of metaphysical views you hold, and if you think it does, then you're doing science *wrong*. It is a *very* Kantian move in that it involves separating the things-in-themselves (metaphysics) from the things-as-they-appear (science). It's definitely not brute materialism.