Naturalism is an epistemological monism, or a claim that being is only known in one way. This still allows us to know math and logic in ways different than the physical world, but only under the assumption that these forms, while objective, are not real beings (an idea with a long history reaching back to Aristotle’s critique of mathematical forms.) Morality also has to be either unreal or Naturalist, though its unreality might be of the sort that the “is-ought” distinction is trying to describe. There are also various attempts to distinguish nature (that is, the real) and the social/cultural, which seems to push this idea of “objective, but not real” about as far as we are ready to let it go.
Comparing different findings from one method, however, is not the same thing as to see the method in the field of possible explanatory tools, and so any epistemological monism will have to build some bridge from the domain in which its outcomes are matters of pride to the domain where the method itself can be evaluated in relation to other methods. The pragmatist criterion seems very attractive here: the method just is justified by its results. One weakness to this approach is that epistemological pluralism does not require that there be method B can compete with method A in A’s own domain, nor does it even require that it give knowledge as extensive, clear, or well-defined as A does. Almost all Medievals were pluralists, but none would argue that the knowledges were equal in any interesting respect.