Epistemology as Metaphysics
We usually teach epistemology as a separate field from metaphysics. On the one hand, there is reality, the subject matter of metaphysics, and on the other there is knowledge of reality, the subject matter of epistemology. It is sometimes said that Descartes made epistemology the foundation of philosophy, ahead of metaphysics, which would only be possible if epistemology were not a species of metaphysics.  But how could it be, given that the world is separate from our knowledge of the world? The thing known is not the same as the knowing of it. However, this way of carving up the terrain ignores an obvious point, namely that knowledge is also something inthe world—part of reality. If the world is the totality of facts, then knowledge is part of the world, because there are facts about knowledge. Given that facts are the instantiation of properties by objects, we can say that knowledge is the instantiation of an epistemic property by a subject. Each of us instantiates many such properties, because we know many things. There are epistemic facts alongside other types of fact. Compare the philosophy of mind: this is not something separate from metaphysics but a branch of it; it is not somehow opposed to metaphysics. Philosophy of mind (as distinct from scientific psychology) is precisely the metaphysics of mind, and is often so described. Well, epistemology is the metaphysics of knowledge (as distinct from cognitive psychology)—the study of one type of property or fact. If we equate metaphysics with ontology (the “study of being”), then epistemology is a branch of ontology, simply because it investigates a region of being—the epistemic region. Thus metaphysics subsumes epistemology.
It is the same with ethics. Ethics is not independent of metaphysics but a department of metaphysics (I am talking about so-called meta-ethics). Moral philosophy is just the metaphysics of morals (as Kant entitled his famous book). It seeks to answer general questions about the status of moral value—whether it is subjective or objective, relative or absolute, a matter of emotion of cognition, etc. Similarly, epistemology concerns itself with such general questions about knowledge: whether we have any, what types of knowledge there are, whether knowledge is the same as true belief, what the nature of justification is, etc. We might helpfully divide the philosophical study of knowledge into three areas to be labeled “practical epistemics”, “normative epistemics” (or “epistemic theory”), and “meta-epistemics”. Practical epistemics deals with specific questions such as whether the belief in God can be justified, or whether we have good reasons to accept Darwinian theory, or whether we really know that global warming is real. These are analogous to the ground floor questions dealt with in practical ethics (abortion, animal rights, capital punishment, etc.). Normative epistemics deals with the general nature of justification (an epistemic norm): is it a matter of consequences, as with pragmatism, or is it constituted by conformity to rules of inference such as induction, deduction, and abduction? This is analogous to normative ethics, which deals with the general notion of right action (giving us consequentialism and deontology). Then there is meta-epistemics, which addresses itself to the analysis of knowledge, the possibility of knowledge (skepticism, epistemic limitation), the objectivity of justification, etc. Just as not all of moral philosophy is rightly described as metaphysics, though some certainly is, so epistemic philosophy is not all metaphysical in nature, though some certainly is. The standard questions of a university course on epistemology are in effect metaphysical questions about knowledge. For example, asking after the general nature of knowledge (the “analysis of knowledge”) is a metaphysical (ontological) inquiry—it wants to know the nature of a certain type of fact. Is knowledge reducible to true belief? Is knowledge constituted by a certain sort of causal connection to the world? Is justification a matter of coherence or indubitable foundations? These are all questions about a certain sort of property, not different in kind from questions about belief or meaning or sensation. We might even say that epistemology is one branch of the philosophy of mind, being concerned with certain attributes of mind (epistemic attributes); and we already know that philosophy of mind is a branch of metaphysics.
Viewing the geography this way is not, as they say, purely semantic, a matter of mere labeling. For including epistemology in metaphysics opens up ways of thinking that might prove helpful in epistemology. For instance, the analysis of knowledge has been confined to specifying conceptual constituents for the concept of knowledge, as with the classic analysis into truth, belief, and justification. But applying apparatus developed in the metaphysics of mind yields other options: what about the idea that the property of knowledge is a simple primitive property that is nevertheless supervenient on truth, belief, and justification (or whatever else needs to be added)? That is, we adopt a non-reductive but dependent view of the property of knowledge. We thus take the concept of knowledge to be non-derivative yet not divorced from other facts about the knowing subject. Knowledge would then resemble goodness as Moore conceived it, or as some metaphysicians view color: dependent but conceptually irreducible. Also, we could treat the topic of epistemic norms as part of a general metaphysical issue concerning norms in nature, as with moral norms and linguistic norms. What we ought to believe is one kind of “fact” that needs to be located in a world of purely natural facts; or it is held not to be a kind of fact at all—depending on your metaphysical views. Naturalizing epistemology is thus like naturalizing ethics or semantics. We can’t really consider the question with respect to epistemology without taking on the broader metaphysical question, construed as such (Quine should have called his much-cited paper “Metaphysics Naturalized” ). Third, the question of skepticism can be recast in ontological terms: do the facts about our reasons for our beliefs necessitate the truth of those beliefs? Just as we can ask whether facts about, say, constant conjunction necessitate (entail) causal facts, so we can ask whether facts about our perceptual reasons for belief necessitate facts about the external world. The brain in a vat scenario seems to show that they do not—there is no such entailment, necessitation, supervenience. The problem then has much the same form as other metaphysical problems: we can’t get one kind of fact to add up to another kind of fact. Truths about the external world always go beyond truths about sensory experience—hence skepticism. Skepticism thus reflects the logical arrangement of facts. It would be different if our reasons for belief actually included the facts we believe (“naïve realism”)—and that is a possible metaphysical view. Again, metaphysics is driving the argument.
If this position is correct, it is impossible to claim that epistemology could be basic in philosophy, if that means more basic than metaphysics. It is metaphysics. What Descartes really did was make one branch of metaphysics more basic than other branches of metaphysics (if we accept the initial claim); more exactly, he made the method of doubt basic in epistemology, which is a branch of metaphysics. He didn’t suppose that epistemology is somehow free from metaphysics—above the metaphysical fray. Indeed, his epistemology is rooted in his metaphysis of mind, because he held that only an immaterial substance could have thought as its essence, which is what knowledge consists in. He didn’t derive his dualism from his epistemology; his view of knowledge rests on a prior metaphysical conception. Whatever knowledge turns out to be, it must be consistent with the fact that the knowing subject is an immaterial being. Nor did anyone else ever make epistemology prior to metaphysics, because that would be to deny that knowledge is an aspect of being, i.e. part of reality. Knowledge is a property that certain objects (subjects) have; and like all properties it has a nature, which can be investigated as such. That is a metaphysical undertaking. We call it epistemology. If someone were to claim that philosophy of mind or philosophy of language were prior to metaphysics, the reply would be the same: these are domains of fact too, and therefore subject to ontological inquiry. Maybe the metaphysics of mind or language could be more basic than other kinds, but they are not more basic than metaphysics in general. It would be the same if someone were to claim that ethics is more basic than metaphysics; the reply would be, “But what about meta-ethics?”
Two metaphysical approaches to epistemology can be distinguished: classical empiricism and Quinean materialism. The ontology of the former includes such items as impressions and ideas, sense data and qualia, while the latter rejects those items and seeks to get by with retinal irritations and the triggering of assent behavior. If someone were to claim that epistemology precedes metaphysics, we could ask him to tell us what kind of epistemic ontology he favors—empiricist or materialist. Then it would be obvious that he already harbors substantive metaphysical commitments. Nowadays people tend to speak of cognitions, data structures, computational operations, informational flow, etc., but again this is metaphysics—claims about what there is. (They might be said to be scientific claims, but then you are adopting a science-based metaphysics.) You can’t get away from ontology in any theorizing, and epistemology is no different. All theories are theories about what there is. The only question is what kind of entity you are going to traffic in, in epistemology and elsewhere. There have indeed been a number of “turns” in philosophy—the linguistic turn, the conceptual turn, the scientific turn, the biological turn, the epistemological turn. But these cannot be characterized as turns away from metaphysics towards some less maddening domain of inquiry, since they are all of them species of metaphysics, i.e. ontologically committed areas of thought. In fact, they all raise thorny metaphysical problems, so they offer no respite from the travails of metaphysics. There is thus no firm ground outside of metaphysics from which to survey the old metaphysical problems. So far from being eliminable metaphysics is inescapable.
Is there any area of philosophy in which metaphysics disappears, no matter whether that area might lay claim to foundational status? What about aesthetics or philosophy of logic or philosophy of science? Brief reflection shows that the answer is no. Aesthetics must face the question of the ontological status of beauty; philosophy of logic must deal with the nature of logical necessity; philosophy of science must reckon with the status of unobservable entities and the nature of laws. Maybe we can do some medical ethics without confronting metaphysical questions (though the question of moral objectivity is never far away), or perhaps legal philosophy (though the nature of law is ultimately a metaphysical question); but philosophy in general is actually permeated with metaphysical questions. The idea that we can avoid metaphysics is a dream born of frustration—pure wishful thinking.
Why is this (elementary) point about epistemology not generally recognized? I think it is because we have a tendency not to think of ourselves objectively—as one element in a wider world. We feel we stand apart from the world. So it is natural for us to assume that our knowledge of the world is not something in the world. In epistemology we study ourselves qua knowing subjects, and so it is easy to think that we are not studying a part of the world—the world that includes us and our knowledge. We think there is us over here and the world over there—me versus it. But this way of seeing things ignores the fact that we are part of the world; and our epistemic states and faculties are just properties of that part. It is true that we can only know the world in virtue of our cognitive faculties, but that doesn’t imply that those faculties aren’t facts of nature among other facts of nature. Once we ascend to a more objective conception of our place in the world, we see that the study of human knowledge is just the study of a certain kind of fact, a kind that concerns the human mind, which itself is just a part of nature. So this aspect of human nature is one kind of being beside others, and so falls within the general theory of being. That theory is just metaphysics or ontology: hence epistemology as metaphysics. 
 Of course, there is a lot of metaphysics that is not epistemology—in no way is metaphysics in general to be assimilated to epistemology. The point is just that epistemology is a branch of metaphysics—subsumption not identity.