If claiming to be a "naturalist" means anything at all, it must mean that one is some sort of metaphysical monist; put the other way around, if "nature" just refers to whatever exists, metaphysically heterogenous or not, it is a vacuous term.
Hume says, "For as to the notion of external existence, when taken for something specifically different from our perceptions, we have already shown its absurdity" (Treatise 1.4.2, Of scepticism with regard to the senses). Before we follow this reference, note that Hume is not, as commonly supposed, some sort of happy sceptic, whose empiricism entails codifying Cartesian scepticism as irrefutable. Very much to the contrary, Hume takes the much more ambitious position that Cartesian scepticism is a pseudoproblem, based on misunderstanding. The very idea of a distinction between "external existence" and "perception" is absurd. Thus the idea that we are "stuck inside our heads," unable to see around our "mental representations," is absurd. The representational theory of mind itself is an absurdity. And if this interpretation of Hume bears textual scrutiny, as I think it does, the whole main trunk of Hume interpretation, sympathetic and hostile, of the past 250 years is spurious root and branch.
This absurdity is shown, Hume claims, at 1.2.6, Of the idea of existence, and of external existence. "Now since nothing is ever present to the mind but perceptions, and since all ideas are deriv'd from something antecedently present to the mind; it follows, that 'tis impossible for us so much as to conceive or form an idea of any thing specifically different from ideas and impressions." This is Berkeleyan. Berkeley, who seems so bizarre to generations of undergraduates who are simply presented with his idealism without enough context, is basically a competent philosopher trying to clean up a mess: if we can't make our way from the "properties of the things-in-themselves" and the "properties of experience," granting that experience is what we have, let's go with experience and leave it at that. A thoroughly empiricist solution to be sure. But not one that warrants the bizarre (Cartesian) metaphysical interpretation to which Berkeley is commonly subjected. The point rather is that the whole discussion, from Descartes to Locke, is a mistake. It is meaningless to talk about some "reality" beyond the reality of experience. This (one must repeat the point to have any chance of overcoming centuries of indoctrination) is not at all the same as saying that there is an external world to which we do not have access, trapped as we are within our experience. "The world," understood in any meaningful way, refers to the world of experience. It is literally inconceivable that there might be a world distinct from experience, or experience distinct from the world. Technically the position is nominalist: "the world" is the name of the category of all experiences. And on that point, Hume, in a footnote to 1.2.6, explicitly refers us to a citation of Berkeley: "A great philosopher (Berkeley)...has asserted, that all general ideas are nothing but particular ones, annex'd to a certain term, which gives them a more extensive signification, and makes them recal upon occasion other individuals, which are similar to them" (1.1.7, Of abstract ideas: the "external world" is an abstract idea of this kind).
The idea is common to scholars of Buddhism. Dogen, the classical sage of Zen, says that experience (the mind) is "the blade that cuts, but does not cut itself." The mind cannot perceive itself (Hume: there is no "self" other than experience itself). Thus there is no Cartesian mystery as to "the world-in-itself" vs. "the world as I experience it." When the distinction breaks down, both sides of the Cartesian dilemma vanish simultaneously: it is equally absurd to refer to "representations" as it is to "mind-independent reality."
I have to go to K-Mart to buy a laundry basket and a dish-drying rack, and take a comforter over to my mother-in-law's house (speaking of reality). Final chunk of argument for this afternoon: it turns out that perceptual states, on Hume's view, are not "copies" of external reality (this has been shown to be absurd). Rather they are states of the body. That is, there is no metaphysical distinction between the mind and the body. It is true enough that when we talk about our "perceptions" we are talking about states of our own bodies; this need not involve us in the concept of "representation." And here's a remarkable outcome: this is Spinoza's view as well.