Looking back over chapter 4 of aime (on the beings of reproduction [rep]) I'm now struck by the description of formalism. I think that this is really the key part of the whole chapter. In fact the whole critique of 'matter' is based around it.
The 'idealist materialism' that Latour roughly delineates is only bifurcated (into material and ideal) if we fail to make another distinction between two kinds of ideality: subjective and objective. If we take these as separate categories (and given the discussion of Modern formalism I think that this separation makes perfect sense) then we have a trifurcated, triangular configuration:
1: objective-ideal 2: subjective-ideal
And the irony of it is that the most important relation in that triangle, for the Moderns, is the one between the two top terms, between the two idealities. It's by establishing that relation that the Moderns believe that they come to dominate matter. It is only by positing that behind all the dirty, complex matter of the actual mountain there is a formal mountain existing in res extensa that one can then conceive of this form being mirrored in the subjective idealist realm of the res cogitans. In order for 2 to 'mirror' 1, 1 must be radically separated from 3. The correspondence theory of truth (the quintessence of Modernism, surely?) only makes any sense if there are two realms of ideality with matter filling in the empty, abstract space.
For all the talk of materialism, matter isn't even the most important aspect of this Modern schema. Modern matter is an artefact of idealism. It undergirds and fills out the arrangement, nothing more. Matter explains persistence because it is unbreakable, eternal, it only shifts form - so it undergirds. And it fills out the schema by being the 'stuff' that has 'adventures through space.' So, matter gives the metaphysic 'substance' in both senses of that word. It gives ideality both persistence and content.
Matter is what is to be dominated and possessed thanks to the marriage of the two idealities made possible by the advent of rational thought - that's the Modern metaphysic and the Modern politic.
Every materialism is also a formalism - the latter is rarely given even a fraction of the attention of the former and yet it is arguably the most important term.