Hume, a thoroughly modern anti-philosophy philosopher, argued that much confusion and intemperate speculation could be got rid of by acknowledging that knowledge, and therefore philosophy, had limits. There are what I call a “soft” and a “hard’ interpretation of Hume. On the soft interpretation, Hume flags an eternal question mark hanging over the limit of experience: we cannot know what lies beyond and we must simply accept that fact. This is Hume the cheerful skeptic, reassuring us that it’s alright that there are things we cannot know and cannot prove. On the hard interpretation, Hume defines knowledge as the output of experience. Belief is only habituation. In Of Miracles, for example, Hume is not telling the reader that Hume does not believe in miracles: he is arguing that the reader does not believe in miracles by virtue of the very definition of “belief.”
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). Hume is not a skeptic whose empiricism entails codifying Cartesian scepticism as irrefutable. To the contrary, Hume takes the position that Cartesian scepticism is a pseudo-problem. An idea that unifies the Treatise is that, contrary to the rationalists’ assertion that logical proof is the paradigm of knowledge, there are in fact no logical proofs for anything we “know” (and Hume is very much focused, as any good epistemologist should be, on the appropriate conditions for the use of the verb “to know”). To “know” something, for Hume, is to be habituated to have a certain expectation of potential future experiences by the regularities of past experiences.
The very idea of a distinction between “external existence” and “perception,” Hume says, is absurd. The idea that we are stuck inside our heads, unable to see around our mental representations, is absurd. At 1.2.6, Of the idea of existence, and of external existence, Hume writes: “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.” It is meaningless to talk about some “reality” beyond the reality of experience since on the empiricist criterion of “meaningful” a statement is significant to the extent that it can be confirmed or disconfirmed on the basis of experience. This is exactly Berkeley’s view, stated in less paradoxical language: the Humean version of the argument that the problem of phenomenal properties is a pseudoproblem hinges, as Berkeley’s version does, on the absurdity of stating that there are physical properties when these are taken to be properties separate from the properties of experience.
This is not at all equivalent to saying that there is an external world to which we do not have access, trapped as we are within the conceptual framework of our representation (remember it is Kant who insists on that): empiricism rules out any such speculation. When Hume says that there can be no proof of the external world he is simply iterating another example of his refutation of Cartesian rationalism: if there are no rational proofs of anything than the word “knowledge” cannot refer to beliefs grounded in rational proof as distinct from 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 cites 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 recall 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).
Perceptual states, on Hume's view, are not “copies” of external reality (this has been shown to be absurd by extension of the absurdity of the concept of “external reality” itself). Rather they are states of the body: the (physical) process of perception has caused a (physical) change to the body (the “impression”). Crucially the impression is not identical to the experience. The formation of the impression is one physical part of the larger physical event of experience. There can be no metaphysical distinction between the mind and the body for Hume, who denies the possibility of any metaphysical distinctions whatsoever (exactly as Berkeley does). When we talk about our “impressions” we are talking about states of our own bodies; this need not involve us in the concept of representation. This is why Hume says that we cannot even assume a numerical correspondence between impressions and “actual objects”: impressions, understood as the physical effects of physical causes, cannot be assumed to perform a representational function in the first place.
Aren’t we discussing qualia here, as distinct from mental representations? Can we sort them out? Yes: there are, on Hume’s view, no representations, but there certainly are experiences. Experiences don’t represent the world; they are constitutive of the world. So the qualities of experience are identical to the qualities of the world, nothing more or less. That is, if it makes no sense to speak of “qualities of the world” if these are meant to be distinct from the qualities of experience then by the same token it makes no sense to speak of “qualities of experience” either.
What remains is to decide how to talk (philosophy often comes down to making decisions about how to talk). Berkeley and Chalmers propose radical solutions: say that the world is constituted by “ideas” rather than by “matter” and say that the qualities of experience are non-physical properties, respectively. However, to accept Berkeley is to reject Chalmers and vice versa, because where Chalmers would codify the metaphysical distinction between mind and matter Berkeley would abolish it. I’m guessing most readers will agree that abolition is a better solution than codification, because codification amounts to simply throwing in the towel and conceding that naturalization is impossible. For myself, I am a staunch abolitionist.
Hume, writing with Berkeley well-digested, develops the essential argument without the outré metaphysical language. Hume collapses the subject-object distinction into the subject as Berkeley did before him. But Hume recognized that once the distinction was collapsed neither of the categories “materialism” or “idealism” made any sense. The distinction simply vanishes. If there are no “physical” properties when these are meant to be separate from “phenomenal” properties the argument works with equal force in the other direction as well. It is on this point that the early 20th century empiricists misinterpreted Hume so grievously, understanding him as a “phenomenalist,” one who holds that we can refer only to “phenomena” as distinct from the “actual” (else why use the word at all?). On empiricism properly understood there is no meaningful (semantic) distinction on which to hang the metaphysical language. This insight is also the crux of the next two versions of the argument, but both are worth considering in detail by virtue of differences in language and emphasis, and to cement the point.