When we speak of nihilism we are apt to think of moral nihilism, the kind of thing discussed in Turgenev’s Fathers and Sons or by Nietzsche or the existentialists. This is the idea that moral values are fictitious, spurious, and non-existent. But the term itself is broader than that, deriving from the Latin “nihil” meaning “nothing”. The OED gives us two definitions: “the rejection of all religious and moral principles, often in the belief that life is meaningless”, and “Philosophy the belief that nothing has a real existence”. The latter is striking suggesting as it does the radical metaphysical position that nothing at all exists. Quite what the scope of the quantifier may be is left up to us, but we may suppose that ontological nihilism is intended, i.e. that no mind-independent entities exist. It would not be denied that thoughts exist. The position I want to consider here is both more and less modest than that: it is the thesis that knowledge does not exist. There is no such thing as knowing: all talk of knowledge is so much fiction, reification, and false objectification. Knowledge is like the unicorn: a mythical entity. Epistemic nihilism is to be distinguished from skepticism, which concerns what is known not the alleged state of knowing itself. Maybe nothing is known, or very little, but the concept of knowledge is a concept in good standing—we know what knowledge would be. There is such a state, but we are seldom if ever in it. By contrast, the epistemic nihilist holds that the state of knowing is a non-existent state—possibly an incoherent one. We should therefore eliminate the concept from our conceptual scheme, or keep it only under strict instructions about how it is to be understood (see below). The epistemic nihilist is like the moral nihilist: both think that the things in question simply lack real existence. There is no such thing as right and wrong, and there is no such thing as knowledge. This is consistent with allowing for the existence of many other things (actions, beliefs); it is specifically moral values and states of knowledge that are declared to be nothing. 
What reasons might be given in support of epistemic nihilism? They are for the most part familiar, but not usually considered as leading to such a radical conclusion. I will merely list them, with the aim of giving the flavor of the position. First, the concept (and therefore the thing) has resisted adequate definition for over 2,000 years, ever since Plato raised the question. We all know that true justified belief fails to add up to knowledge proper (Russell, Gettier). Even now we cannot say what knowledge is, despite our best efforts. The nihilist takes this to show that knowledge is nothing definable: the reason it can’t be defined is that it has no reality to be defined. No one is ever in such a state (even when the skeptic has been silenced). Second, there are deep puzzles about knowledge, also ancient: we can’t say how a priori knowledge is possible, and there are problems about the nature of empirical knowledge.  How can we come to know things by pure reason—what kind of process is this? What explains it? And why is it that the world can only be known by sense experience? Thus some have supposed that so-called a priori knowledge is not really knowledge at all, since it concerns only tautologies or human conventions (there are no real propositions to be known in this way). And others have doubted that experience can ever add up to genuine knowledge: knowledge must be more than experience alone, but what is that more? Does knowledge have a foundation in experience, and how does that work exactly? Is knowledge simply coherence of belief? But how does mere coherence suffice for truth (a requirement of knowledge)? We thus cannot secure a prioriknowledge or a posteriori knowledge. Both are profoundly problematic. The epistemic nihilist sees in these failures a reason to doubt that the concept of knowledge is a workable concept—that it denotes anything real with which we have to come to terms. We may as well just get rid of it. Third, skepticism shows that the concept has no actual application—and what is the point of a concept that never applies to anything? Even when it does apply (e.g. knowledge of our own subjective experience) it has only limited application: in most of its uses it is falsely applied. There is clearly something amiss with a concept like this. Surely it must have been introduced in error, before its consequences were thought through. It is just a hopelessly shambolic concept, containing the seeds of its own destruction. Why bother keeping a concept that leads to such outlandish results? Why not declare it a would-be concept sorely in need of a real-world correlate? It simply doesn’t stand for anything real. Fifth, there are serious problems about what the state of knowledge could be. Here we may compare knowledge with meaning: is there any fact of meaning something by our words—what could constitute meaning?  Similarly, is there any fact of knowing—what could constitute such a fact? Meaning is not an introspectable state of consciousness, nor is it a disposition, nor a brain state: so what is it? What determines whether we mean one thing or another by our words? Similarly, knowledge is not an introspectable state of consciousness, nor is it a disposition (there are always performance errors), nor a brain state: so what is it? What determines whether we know one thing rather than another? It seems to be an infuriatingly elusive kind of fact, like meaning. Hence we get indeterminacy claims and suggestions of non-factuality. Maybe we can salvage talk of knowledge by invoking non-standard semantic theories—as with criteria-based assertibility conditions theories—but then we have already conceded that the world contains no actual state of knowledge. The word “know” has a use, but it denotes nothing real. This is the analogue of anti-realist assertibility conditions theories of meaning. The epistemic nihilist may grudgingly accept such a diluted picture of knowledge for the sake of giving the word “know” a role in our language games, but still insist that nothing real is being designated. The word “know” is like the word “trustworthy”—expressing an attitude we can have towards certain individuals, not denoting an objective property of them. We thus give a pragmatic or expressive account of such talk without supposing that anything objectively real is going on. Or we might simply abolish the whole knowledge language game as so much error and myth. In either case we need not worry that we are failing to grasp the nature of something real—for there is nothing real there to grasp. Knowledge has no nature, no real essence, and no objective constitution—any more than unicorns do (or phlogiston or fairies). It is not as if we discovered the state of knowledge by means of diligent scientific observation. We just find ourselves with the concept, justifiably or not. Perhaps concepts like belief, truth, and justification can function as criteria of assertion for knowledge sentences, but they are not to be construed as denoting real constituents of a language-independent fact of knowing. We thus adopt an “error theory” of knowledge talk, like error theories of talk of meaning or moral value. We thereby extend nihilism into new territory.
The advantage of taking this route is that it dissolves problems, as pragmatic and expressive theories are intended to do. We find ourselves burdened with a concept that is riddled with conundrums, mysteries and puzzles, and we summarily dismiss them by declaring outright non-existence. All the classic problems of theology disappear once God is declared dead. Similarly for the problems of ethics, given that there is no such thing as right and wrong; same for meaning, if meaning proves intractable; same for consciousness, if consciousness remains mysterious; and same for knowledge, if knowledge presents irresoluble difficulties. Come to think of it, isn’t the concept of knowledge tied suspiciously closely to religious ideas? God is defined as an all-knowing being, but according to religious nihilism there is no such being. Popes and priests are traditionally supposed to be epistemic authorities (revelation etc.), but that is a preposterous presumption. Is the concept of knowledge really an insidious way to reinforce social hierarchies and foster superstitions? Who possesses real knowledge and who doesn’t?  Much the same has been said of moral value, which is similarly divisive. So the concept of knowledge might be thought to have dubious historical roots as well as being internally defective. The epistemic nihilist proposes to do away with knowledge as part of serious ontology, perhaps allowing such talk a limited pragmatic role (there certainly can’t be any science of it). Belief, yes, perhaps even justification, but not knowledge—not that old shibboleth. We needn’t keep trying desperately to find a definition for it, or figure out how it is possible, or what justifies it, or how it is related to experience, or what varieties of knowledge there might be, or whether there is any knowledge at all, or whether ethics (say) is an example of knowledge. Epistemology gets reconfigured, with knowledge losing its central place, or any place. True, the new type of knowledge-free epistemology is radically different from the old, but so was post-Copernican astronomy radically different from what went before, or Darwinian biology, or Einsteinian physics, or secular morality, or democratic politics. In order to make these great advances we often need to reject the existence of things hitherto taken for granted (divine design, vital spirits, the ether, the immortal soul, God-given commandments), but this can lead to exciting new vistas. What will epistemology look like without the obsession with knowledge? The epistemic nihilist boldly goes where no epistemologist has gone before. She points out the theoretical advantages, and the removal of troublesome questions, and the easing of our abiding sense of futility. We will still have good thinkers and bad, reliable informants and unreliable ones, real science and pseudo-science; we just stop characterizing all this with the archaic concept knowledge. The epistemological landscape will not be rendered depressingly deserted; it might even be fuller and healthier, with the air easier to breathe. The nihilist with respect to Hell, the Devil, and demons is a welcome presence, bequeathing to us a far healthier spiritual world; the nihilist with respect to knowledge hopes to achieve a similar result. She sees herself in a positive light, not as a spoiler and naysayer, but as a bringer of good news not bad. Just think: you will never have to berate yourself again for not knowing something! We will still have perception and memory, belief and inference, good reasons and bad, but we won’t need to aspire to something called “knowledge”—whatever that might be exactly. For the human cognitive system is never in a state that can be so characterized. The word “know” is the analogue in psychology and philosophy of “vital spirits” in biology. Knowledge has no place in science, and no place in common sense either. 
 In fact the two issues are not unconnected, since knowledge is commonly regarded as a normative concept—it is what belief aspires to be. Knowledge is often included on lists of things that are good intrinsically. The moral nihilist may thus have knowledge in his sights too, as dubiously value-laden.
 I am alluding to Kripke’s Wittgenstein on Rules and Private Language. We could, in fact, rephrase Kripke’s discussion of the “skeptical paradox” of meaning in terms of knowledge of meaning: is there a fact of the matter about whether I know that “+” means addition? Does John know that emeralds are green or is it that he knows that emeralds are grue?
 The hero (victim) in Nabokov’s Invitation to a Beheading is scheduled to be executed for the crime of “gnostical turpitude”. What is heresy but claiming to know that what the church authorities say is knowledge isn’t knowledge? Religion is an epistemic battleground.
 The case of knowledge of language is interesting: do we know the grammar of our language? That has always seemed problematic if we are working with the concept of knowledge that is regularly analyzed as true justified belief (plus some). Mastery of language certainly involves ability, competence, and internal representational structure: but does it involve knowledge? We can simply dispense with that question once the concept of knowledge has been banished from serious discourse; and doesn’t this seem particularly appropriate where linguistic mastery is concerned? We can also retain such notions as knowing-how and knowing-whom: it is knowledge of facts (knowledge-that) that has caused all the trouble. Have we illicitly extended the concept of knowledge from its unproblematic uses (“he knows how to play piano”) to create a concept that is obscure at best (“he knows that there is a table there”)? Not to mention “he knows that 2+2=4” and “he knows that genocide is wrong” and “he knows that he is in pain”. These all raise red flags, and the epistemic nihilist has an explanation of why: we are throwing the word “know” around recklessly and don’t really know what we mean. We do better to keep the word “know” under strict supervision and not let it spread to places where it doesn’t belong.