1 What does the verb ‘know’ mean?

Philosophers have spent two and a half millennia failing to define “know” satisfactorily. That failure is not a sign of philosophy’s weakness. It is a sign of how hard the question is.

1.1 The Tripartite Definition

Plato’s dialogue Theaetetus (c. 369 BCE) proposes what has become the standard starting point. Knowledge, suggests Socrates, is justified true belief: to know something, you must (1) believe it, (2) it must be true, and (3) you must have adequate justification for your belief. This is often abbreviated JTB.⁴

Three conditions, three separate questions:

• Belief: Can you know something you don’t believe? Could you know the answer to a question and still write the wrong answer on a test? (Some philosophers say yes — you might “know” in some implicit sense while consciously believing the opposite.)
• Truth: Can you know something that turns out to be false? If a geography teacher “knows” that the capital of Australia is Sydney, and they are wrong, were they ever knowing, or only believing?
• Justification: What makes a belief justified? Perception? Memory? Testimony? Inference? This is where much of the real work in epistemology happens.

The tripartite definition has the advantage of capturing our intuitions: hunches are not knowledge (no justification), lucky true guesses are not knowledge (no justification), false beliefs are not knowledge (not true). It held, more or less, until 1963.

1.2 The Gettier Problem

In 1963, Edmund Gettier published a three-page paper — “Is Justified True Belief Knowledge?” — that changed the field overnight.⁵ He gave two counterexamples showing that you could have a justified true belief that is, intuitively, not knowledge.

The classic reconstruction: Smith and Jones are applying for a job. Smith has strong evidence (the boss told him directly) that Jones will get the job. Smith has also counted the coins in Jones’s pocket: ten coins. Smith therefore believes: The man who will get the job has ten coins in his pocket. This belief is justified. But — unbeknownst to Smith — Smith himself gets the job. And, by coincidence, Smith also has ten coins in his pocket. So Smith’s belief is true. And it was justified. But it was not knowledge — it was true for entirely the wrong reasons.

The philosophical response since 1963 has been to add a fourth condition, change what we mean by justification, or abandon JTB altogether. None of the proposed solutions has won consensus. The problem remains open.

What it tells us: Justification can be accidentally disconnected from truth. Being right for the wrong reasons is not the same as knowing.

Return now to Hillsborough. Most of what is morally outrageous about the case — Duckenfield’s lie about the gate, The Sun’s smear, the 116 amended statements — is not what is philosophically interesting in the JTB / Gettier frame. Lying is a separate epistemic vice; JTB has no trouble classifying assertions known by the asserter to be false. The harder pattern sits underneath the cover-up. Among officers who genuinely believed what they reported, the case fits the JTB framework awkwardly in more than one way. Some officers asserted what was simply false (most pointedly: the claim that fans, not Duckenfield’s order, had opened gate C) — the truth condition was not met, and JTB classifies these correctly as non-knowledge. Others were strictly correct about peripheral facts (that some fans had been drinking; that fans had pressed forward at the gates) while their account of the central causal story was wrong; for those peripheral beliefs, the justification — rumour, group consensus, retrospective fitting — was not tracking the truth, and the structure resembles Gettier’s: true belief produced by a process unconnected to what made it true. (Strictly, only the second sub-case is a Gettier case; the first is a more mundane JTB failure; the Duckenfield lie is neither.) The courtroom was not asking a piece of philosophy by accident — the distinction between knowing and merely stating the true thing has legal consequences.

1.3 Knowledge, Belief, Opinion

The Greek words Plato uses are instructive: episteme (knowledge), doxa (belief or opinion), aletheia (truth). The distinction matters. Opinion can be true or false; knowledge, if we accept the tripartite definition, must be true. But the intuitive difference goes deeper: knowledge feels stable in a way that belief doesn’t.

Compare:

• “I believe it will rain tomorrow.”
• “I know that water is \(\mathrm{H_2O}\).”
• “In my opinion, the painting is beautiful.”

These feel categorically different. But are they? Wittgenstein, late in his career, wrote On Certainty (1969) to probe the edges of knowledge — things so basic we cannot doubt them, even if we cannot prove them. “My name is Ludwig Wittgenstein” is not something Wittgenstein knows in the sense of having arguments for it — it is a hinge on which all his other thinking depends.

“At the foundation of well-founded belief lies belief that is not founded.” — Ludwig Wittgenstein, On Certainty, §253⁶

1.4 Knowledge by Acquaintance vs. Knowledge by Description

Bertrand Russell drew a distinction (in The Problems of Philosophy, 1912) that cuts across the JTB debate:⁷

• Knowledge by acquaintance: direct, unmediated contact with something — I know the colour red by seeing it; I know pain by feeling it.
• Knowledge by description: knowing about something through propositions — I know that Julius Caesar was stabbed on the Ides of March without having witnessed it.

Russell’s point: almost all our knowledge of the world is knowledge by description. We are rarely acquainted with the facts themselves. This raises a question about the reliability of the chain of description — testimony, books, memory.

The distinction is worth knowing as Russell drew it, but worth knowing too that Russell himself abandoned it. In the unpublished Theory of Knowledge manuscript of 1913, under direct pressure from Wittgenstein, he gave up acquaintance as a primitive epistemic relation.

Contemporary epistemology has, with rare exceptions, followed him out. McDowell’s disjunctivist account regrounds the idea that perception puts us in direct cognitive contact with the world, without needing a Russellian “acquaintance” relation to do the metaphysical work; the work building on it has done the same.⁸

1.5 Questions to Argue About

• If you cannot know you are not dreaming right now, can you know anything about the external world?
• Is there a difference between knowing how (to ride a bike) and knowing that (Paris is the capital of France)? Gilbert Ryle called this the distinction between procedural and propositional knowledge.⁹ Can the JTB definition handle both?
• Can a computer know something? What would it lack, if anything?
• Is the Gettier problem a devastating objection to JTB, or just a quirky edge case that doesn’t matter in practice?

2 Am I a brain in a vat?

In 1641, René Descartes sat by a fire and decided to doubt everything he could possibly doubt. He wasn’t being dramatic. He was trying to find a foundation for knowledge so solid that nothing could undermine it. His method — radical scepticism — produced one of the most famous arguments in the history of philosophy, and one of the most unsettling questions you can ask yourself on an otherwise ordinary Tuesday.

2.1 The Method of Doubt

Descartes begins Meditations on First Philosophy by noticing that his senses have deceived him before. Things look different sizes depending on their distance. Straight sticks appear bent in water. If the senses can deceive sometimes, can he trust them at all?

He escalates. Perhaps he is dreaming right now. Everything he experiences might be a dream. You’ve had dreams that seemed real. How do you know this isn’t one?

He escalates further. What if an all-powerful evil demon (malin génie) is deceiving him about everything — not just his senses, but even mathematics? Could 2+2=5 be a deception planted by this demon?

“I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me.” — René Descartes, Meditations on First Philosophy, Meditation I¹¹

This is not paranoia. It is a philosophical method: if a belief could possibly be false under any scenario, however far-fetched, it cannot serve as a foundation for knowledge. Descartes is testing what survives absolute doubt.

2.2 The Brain in a Vat

The contemporary version of Descartes’ evil demon is the brain-in-a-vat scenario, articulated by Hilary Putnam in Reason, Truth and History (1981).¹² Imagine that you have always been a disembodied brain in a laboratory, kept alive in a vat of nutrients, with all your sensory experiences generated by a supercomputer. Everything you have ever seen, touched, felt, and thought has been a simulation. How would you know?

The scenario is logically coherent. There is no internal contradiction in the idea. If it is coherent, then for all you know, it might be true. And if you can’t rule it out, how can you claim to know anything about the external world?

This is external world scepticism: the view that we cannot have knowledge of the world beyond our own minds. Descartes himself didn’t accept this as the final word. He thought he could escape it. The escape route he took — proving God’s existence to guarantee the reliability of his faculties — is not widely accepted today, though it has had serious twentieth-century defenders. Alvin Plantinga’s reformed epistemology argues that belief in God can be properly basic, and that without something like a divine guarantee, the reliability of our cognitive faculties is itself an unsupported assumption — his evolutionary argument against naturalism contends that unguided evolution gives us no reason to expect our belief-forming faculties to track truth at all.¹³ Whether one accepts the Plantingian reply or not, the point is that the Cartesian move is not refuted by the modern fashion against it; it is contested.

2.3 The Cogito

At the bottom of the doubt, Descartes finds something he cannot doubt:

“I think, therefore I am.” (Je pense, donc je suis) — René Descartes, Discourse on the Method, Part IV¹⁴

The Latin formula cogito, ergo sum appears in Descartes’ Principles of Philosophy (1644, Part I, §7), not in the Meditations. In the Meditations on First Philosophy (the later, more rigorous work), the formulation is different: “I am, I exist” — ego sum, ego existo — which Descartes says “is necessarily true whenever it is put forward by me or conceived in my mind” (Meditation II).

The act of doubting itself proves there is a doubter. Even if the demon is deceiving me, there must be a me being deceived. The very act of thinking — including doubting — proves existence.

Descartes treats this as the unshakeable foundation: I exist as a thinking thing. Everything else is rebuilt from here.

But the cogito raises its own problems:

• Does “I think” already smuggle in the assumption of an “I”? Lichtenberg argued that all Descartes can know is that there is thinking, not that I am doing it.¹⁵
• If the cogito is the only certainty, it is remarkably thin. What can you build on “I exist as a thinking thing”?
• Hume later argued that when he introspected to find the “I”, he never caught anything — only a bundle of perceptions. There is no unified self.¹⁶

2.4 Responses to Radical Skepticism

Philosophers have not simply accepted that we know nothing — this would be a strange response from people whose entire professional existence depends on the possibility of knowing things. Several responses follow. The textbook habit is to list them as a buffet; this obscures that they are doing very different things. Some try to refute the sceptic on the sceptic’s own terms (Moore, Putnam’s externalist argument). Some try to change the subject — to show, with Wittgenstein, that the sceptic’s question is not the question it appeared to be. Some accept and deflate the sceptic’s conclusion (the pragmatists). Barry Stroud and Michael Williams have argued, in different ways, that these are not interchangeable moves and that the sceptic’s apparent grip depends on theoretical commitments we are free to reject.¹⁷

Moore’s common sense (G.E. Moore, “Proof of an External World”, 1939): Moore held up his hand and said: “Here is one hand. Here is another. Therefore external things exist.”¹⁸ This seems ludicrous as a philosophical argument, but Moore’s point was that the certainty of the external world is more certain than any philosophical argument against it. If philosophy leads to the conclusion that hands don’t exist, the conclusion is wrong, not the hands.

Wittgenstein on hinges: In On Certainty, Wittgenstein argues that some beliefs function as hinge propositions — they are not things we test against evidence, but the framework within which testing happens at all. “The Earth has existed for a long time” is not a hypothesis I entertain. It is a hinge. Doubting it isn’t possible inside a normal form of life. The sceptic’s mistake is demanding that hinges be supported by something else.

Externalism (Hilary Putnam): Interestingly, Putnam — who introduced the brain-in-a-vat scenario — also argued that if you were a brain in a vat, your word “vat” wouldn’t even refer to real vats (since all your experience of “vat” comes from the simulation). This is a subtle semantic argument that the scenario may be self-undermining.

Pragmatism: William James and John Dewey argued that the question “am I really perceiving the external world?” is meaningless if it makes no practical difference. The sceptical hypothesis, by design, makes no predictions different from ordinary realism. A proposition with no practical consequences is no proposition at all.¹⁹

Zhuangzi’s butterfly dream (Zhuangzi, Book II, “Discussion on Making All Things Equal,” ~4th c. BCE): “Once Zhuangzi dreamt he was a butterfly, a butterfly flitting and fluttering around, happy with himself and doing as he pleased. He didn’t know he was Zhuangzi. Suddenly he woke up and there he was, solid and unmistakable Zhuangzi. But he didn’t know if he was Zhuangzi who had dreamt he was a butterfly, or a butterfly dreaming he was Zhuangzi. Between Zhuangzi and a butterfly there must be some distinction! This is called the Transformation of Things.”²⁰ Zhuangzi raises the same scenario Descartes will raise two thousand years later but draws a different conclusion: not “doubt everything until something certain remains,” but “the demand for a vantage-point above the dreaming and the waking is itself the mistake.” On the Zhuangzian reading the sceptic and Moore are both committed to a perspective from which the question can be settled — and the genuine philosophical move is to notice that no such perspective is available. Whether this is closer to Wittgenstein’s hinge-position or to a third option that neither of the Western moves names is itself worth arguing about.

2.5 What is Left to Know?

Descartes’ inquiry doesn’t leave us with nothing. It leaves us with a precise map of what we can and cannot justify:

• The existence of our own mental states (certainty)
• Mathematical and logical truths (highly certain, but vulnerable to demon scenarios)
• The existence and nature of the external world (not directly justified from the inside)

The limits of Cartesian doubt mark the boundary between knowledge that survives the demon scenario (one’s own mental states, at minimum; perhaps mathematics; perhaps the cogito) and knowledge that does not (the external world, other minds, the past as remembered). That is a substantive map, not a consolation.

2.6 Questions to Argue About

• Could you ever prove — not just assume — that you are not a brain in a vat?
• Is Moore right that common sense certainties should override philosophical arguments? Or does that just refuse to engage with the question?
• Descartes thought the cogito was the one thing immune to doubt. Is it? What assumptions does it rely on?
• If we cannot know whether we’re in a simulation, should that change how we live? Or is the question practically meaningless?

3 What is truth?

“What is truth?” — Pontius Pilate asked the question and didn’t wait for the answer. He may have been wiser than he looked. Everyone wants to know the truth. Politicians say they’re speaking it. Scientists claim to discover it. Courts try to establish it. But what is truth? The answers are genuinely incompatible.

3.1 The Correspondence Theory

The oldest and most intuitive theory: a statement is true if and only if it corresponds to reality. To say “it is raining” is true just in case it is, in fact, raining.

Aristotle stated this with admirable clarity:

“To say of what is that it is not, or of what is not that it is, is false, while to say of what is that it is, or of what is not that it is not, is true.” — Aristotle, Metaphysics, Book IV, Chapter 7²⁴

Truth is about the world fitting the words. A map is accurate when it corresponds to the terrain.

The problems:

• What are the “truth-bearers”? Sentences? Propositions? Thoughts? In different languages, different sentences express the same truth — so the truth-maker must be something language-independent.
• What is the “correspondence relation” exactly? It can’t be resemblance (the word “cat” doesn’t look like a cat). Correspondence turns out to be difficult to cash out precisely.
• How does correspondence work for abstract truths? “7 is prime” corresponds to what, exactly, in physical reality?

3.2 The Coherence Theory

Rather than asking whether a belief corresponds to external reality, the coherence theory says: a belief is true if it coheres with a system of beliefs. Truth is internal consistency within a web.

This sounds strange, but it has a serious motivation: we never have direct access to reality independent of our beliefs. All our evidence for the world comes through our perceptual, conceptual, and theoretical frameworks. There is no “view from nowhere.” So the only available criterion of truth is whether beliefs hang together.

F.H. Bradley and later Bernard Bosanquet (British Idealists, late 19th century) developed this view.²⁵ It is also reflected in Neurath’s “ship” metaphor: we are always rebuilding the ship of knowledge while at sea. We cannot put every plank on dry land at once.²⁶

The problem: Two incompatible and internally coherent belief systems could both be “true” by this criterion. The theory seems to give up on the idea that truth is uniquely tied to reality.

3.3 The Pragmatic Theory

William James (1907) proposed a radical alternative: a belief is true if it works — if believing it is useful, if it “pays” in terms of practical experience.

“The true is only the expedient in our way of thinking, just as the right is only the expedient in our way of behaving.” — William James, Pragmatism, Lecture VI²⁷

This has a democratic, anti-elitist feel: truth is not some mystical correspondence with a Platonic realm, but a practical instrument for navigating experience. The truth of “water is \(\mathrm{H_2O}\)” consists in the countless successful predictions and interventions that belief enables.

The problem: It seems possible for a false belief to be useful. Believing you are invincible might help you win a fight (briefly). And some truths seem useless or actively harmful in the short term. Is truth really just usefulness?

3.4 Tarski’s Semantic Definition

The 20th century produced a more technically precise approach. Alfred Tarski, in “The Concept of Truth in Formalized Languages” (1933), proposed the semantic definition of truth:²⁸

“Snow is white” is true if and only if snow is white.

This seems trivially obvious — almost a joke. But Tarski’s point is that any theory of truth must at minimum satisfy this schema: the sentence on the left is true iff the state of affairs on the right obtains. He called this the T-schema. What makes it powerful is that it prevents paradoxes like the Liar by carefully distinguishing between a language and the meta-language we use to talk about it.

Tarski’s definition is deflationary: it doesn’t tell you what truth is beyond this biconditional. Modern deflationary (or minimalist) theories of truth argue this is all that needs to be said — there is no deep metaphysical property of “truth” beyond what the T-schema captures.

3.5 The Liar Paradox and Self-Reference

[VERIFY: unanchored — suggest anchoring Tarski hierarchy claim to Tarski, “The Concept of Truth in Formalized Languages” (1933), repr. in Logic, Semantics, Metamathematics (Oxford 1956), §1; the Epimenides version traces to Titus 1:12, but the paradox proper is usually attributed to Eubulides of Miletus (Diogenes Laertius II.108)]

Any discussion of truth must confront the Liar: This sentence is false.

If the sentence is true, it is false. If false, it is true. The paradox was known to the ancient Greeks (attributed to Epimenides: “All Cretans are liars,” supposedly said by a Cretan). It shows that unrestricted truth-talk leads to contradiction.

Tarski’s response: a language cannot contain its own truth predicate without contradiction. We need a hierarchy of languages. This is technically satisfying but philosophically strange: it means there is no single language in which we can say everything.

3.6 Questions to Argue About

• Can there be more than one “true” account of an event? (Multiple witnesses, different but equally accurate perspectives?) Is this coherentism, or just multiple truths?
• Is the pragmatic theory of truth just confusing truth with usefulness? Or does James have a point?
• Does it matter, for practical purposes, which theory of truth is correct? Or is this a purely academic dispute?
• If the Liar paradox cannot be fully resolved, does that mean logic has limits? (Preview of the Logic unit on Gödel.)

4 Can we trust our senses?

Look at a stick standing in a glass of water. It bends at the surface. The history of philosophy and psychology is two and a half millennia of asking what to make of that. The question is not whether perception can go wrong — it can — but what that implies for knowledge.

4.1 The Müller-Lyer Illusion

Look at two lines of equal length, one ending in inward-pointing arrowheads (<–>) and one in outward-pointing arrowheads (>–<). The second looks longer. Measure them: they are identical. Now look again. It still looks longer. This is the Müller-Lyer illusion, described by Franz Carl Müller-Lyer in 1889.³⁰

Our perceptual systems are not passive recorders; they are active interpreters, using context, angles, and prior assumptions to construct our experience of the world. The brain “fills in” and “edits” constantly. The perception you have is never a raw image — it is a processed representation.

4.2 Russell’s Table

Bertrand Russell opens The Problems of Philosophy (1912) by asking you to look at a table. You might say the table is brown. But the colour you see depends on the lighting. The shape looks different from different angles. The texture changes depending on your distance from it.

“It becomes evident that the real table, if there is one, is not the same as what we immediately experience by sight or touch or hearing.” — Bertrand Russell, The Problems of Philosophy, Chapter 1³¹

Russell distinguishes between sense-data (what we directly experience — patches of colour, sounds, sensations) and physical objects (the supposed real things causing those experiences). We never directly perceive physical objects. We only ever perceive sense-data.

Russell’s distinction creates the problem of the external world: if all I ever directly experience is my own sense-data, how do I know there is a world of physical objects behind it? How do I know the table persists when no one is looking?

George Berkeley (1685–1753) drew the radical conclusion: esse est percipi — to be is to be perceived. There are no physical objects independent of perception; there is only God and minds and their ideas.³² Samuel Johnson, walking with Boswell in 1763, kicked a large stone and said “I refute it thus.” Berkeley’s defenders reply that the kick produces a sensation of pain — which Berkeley never denied — but that nothing in the kick proves there is anything behind the sensation. Berkeley’s critics reply that mind-independent objects are the simplest hypothesis explaining the regularity, intersubjectivity, and predictability of perception (why do unwatched apples rot the way they do? why do other observers report the same readings I do, even when their interests would have inclined them to report otherwise?). Berkeley’s own reply to that — that God’s perception sustains the world in our absence — is the move on which the whole position stands or falls. Whether the position can be refuted without already assuming the mind-independent existence the position denies is one of those disputes that survives every attempt to settle it.

4.3 Locke’s Primary and Secondary Qualities

John Locke (An Essay Concerning Human Understanding, 1689) offered a more moderate response. He distinguished:³³

• Primary qualities: features of objects that are genuinely in the objects themselves — size, shape, number, motion, solidity. These ideas in our minds resemble how things really are.
• Secondary qualities: features that exist in the object only as powers to produce sensations in us — colour, sound, taste, smell, temperature. The redness of a tomato is not in the tomato in the way its spherical shape is. The redness is how the tomato’s surface reflectance hits our visual system.

Modern science reflects this split: physics describes the world in terms of mass, charge, and position — not colour, taste, or beauty. Those are things minds add.

The problem with Locke: Berkeley immediately objected that we have no reason to trust primary qualities either. If colour is in the mind, what makes shape any different? The “resemblance” between our idea of shape and the shape itself cannot be verified, since we can only compare ideas with ideas, never ideas with things-as-they-are.

4.4 The Reliability of Perception

Contemporary philosophy of perception divides into several camps:

• Naive realism (direct realism): we directly perceive physical objects, not intermediaries. The stick in water is straight; we just misjudge it.
• Representationalism: we perceive internal mental representations of the world. The question is whether those representations are reliable.
• Disjunctivism: genuine perception (of a real object) and hallucination are fundamentally different kinds of mental states, even if they feel the same from the inside.

The trichotomy is a useful first map, but it can mislead. Disjunctivism is not a “third way” between naive realism and representationalism in the manner of a compromise; it is a structural disagreement about what kind of state perceiving even is.

In “Criteria, Defeasibility, and Knowledge” (1982), McDowell argues that the sceptic’s argument trades on a hidden assumption — that perception and hallucination share a “highest common factor” — and once that assumption goes, the argument loses its grip. So which view is right is not just a question about perception; it is downstream of how the sceptical problem is set up in the first place.³⁴

Perception is strongly shaped by expectation, culture, and attention. Classic psychological experiments show we miss things we’re not looking for (the gorilla in the basketball video — Simons & Chabris, 1999).³⁵ We see what we expect to see.

4.5 Questions to Argue About

• If illusions persist even when we know the truth, what does that say about the relationship between perception and belief?
• Locke says primary qualities are in the object; secondary qualities are in the mind. Is this distinction stable? Can you defend it against Berkeley’s attack?
• Does it matter for daily life whether we are directly perceiving the world or representing it? Or is this a distinction without a practical difference?
• If different species perceive the world very differently (dogs smell what we can’t; bats echolocate), which perception is “correct”? Is there a fact of the matter about what the world “really” looks like?

5 Why should I believe what others tell me?

Take an honest inventory. You “know” that Antarctica exists, that Caesar crossed the Rubicon, that Sirius is a star, that your birthday is the date written on your passport, that the medication on the chemist’s shelf is what the label says it is. Almost none of this knowledge is yours by direct acquaintance. You have it because somebody told you — a teacher, a parent, a textbook, a stranger in a uniform, an anonymous Wikipedia editor, a journalist you have never met. Strip out everything you accepted on someone else’s word, and what survives would not get you through breakfast. The question is whether testimony — being told that p — is a genuine source of knowledge in its own right, on a par with seeing it for yourself, or whether everything you took on trust is, strictly speaking, a lower-grade currency that needs to be earned back by independent inference.

5.1 Testimony as a Source

Jennifer Nagel observes that “many of our prized possessions come to us second-hand.”³⁷ The technical term is testimony: someone tells you something, in speech or writing, and the content of what they say plays a special role in what you take from the exchange. (You overhear a friend say “I have a hoarse voice” and hear that the voice is hoarse — that is perception. You read in a newspaper that there has been an earthquake in Turkey — that is testimony.) Testimony so understood is the load-bearing source for almost everything we know about history, geography, science we did not personally do, and the lives of everyone we have not personally observed. Robert Audi puts the structural point bluntly: “If perception, memory, consciousness, and reason are our primary individual sources of knowledge and justification, testimony from others is our primary social source of them.”³⁸ The philosophical question is whether testimony is another basic source — like perception — or whether it is at best a derivative source, parasitic on the others.

5.2 Reductionism: The Hume Position

The reductionist holds that there is no special epistemic credit attached to being told. When you accept what someone says, you must (in principle) be able to back the acceptance with independent evidence about the speaker’s reliability — past track-record, internal coherence, plausibility against your own observation. Testimony reduces to perception, memory, and inference; it adds nothing of its own. The classical reductionist text is Hume’s Enquiry, §X “Of Miracles,” where he is forced to defend the position in the hardest case — testimony to a violation of natural law — but states the underlying epistemology as a general claim:

“A wise man, therefore, proportions his belief to the evidence. […] our assurance in any argument [from testimony] is derived from no other principle than our observation of the veracity of human testimony, and of the usual conformity of facts to the reports of witnesses.” — David Hume, Enquiry Concerning Human Understanding §X.4–5³⁹

Strong consequence (Hume’s main target): “no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish.”⁴⁰ Locke had taken a similar but more general line a century earlier: testimony, however well-supported, falls short of knowledge because it is at best probable, never certain. Strictly speaking, on the Lockean account, you do not know where you were born, that George Washington existed, or that Antarctica exists — you have only “highly probable opinion” about all three.⁴¹

5.3 Anti-Reductionism: The Reid Position

The anti-reductionist holds that testimony is a basic source — like perception — operating with a default of acceptance, not a default of suspicion. We do not, and could not, derive trust in others’ words from prior independent evidence; we begin with credulity, and reasons are required to reject a piece of testimony, not to accept it. The locus classicus is Thomas Reid’s “principle of credulity”:

“The wise and beneficent Author of nature, who intended that we should be social creatures, and that we should receive the greatest and most important part of our knowledge by the information of others, hath, for these purposes, implanted in our natures two principles that tally with each other. […] Another original principle implanted in us by the Supreme Being, is a disposition to confide in the veracity of others, and to believe what they tell us. […] we shall, for want of a more proper name, call this the principle of credulity. It is unlimited in children, until they meet with instances of deceit and falsehood: and it retains a very considerable degree of strength through life.” — Thomas Reid, An Inquiry into the Human Mind, Chapter VI, Section 24⁴²

The most influential modern argument for the Reidian position is C. A. J. Coady’s: pure reductionism is self-undermining. To assemble the case for trusting testimony in general, one would need a vast independent track-record — but most of one’s evidence about that track-record is itself testimonial (history books, reports of others’ observations, experimental results one has not personally re-run). The reductionist programme cannot get off the ground without already trusting the source it claims to reduce.⁴³ The anti-reductionist conclusion: testimony is on epistemic par with perception. Default acceptance is rational; the burden of proof is on doubt.

5.4 Testimonial Injustice

A separate but connected pathology, identified by Miranda Fricker (Epistemic Injustice, 2007), sits inside the Reidian default itself. Even where credibility should be granted, hearers withhold it on the basis of prejudice about the speaker’s identity. Fricker calls this testimonial injustice: a wrong done to someone in their capacity as a knower.⁴⁴ Her central example: a Black man stopped by police in 1970s America gives an account of events; he is disbelieved not because his account is implausible but because his race activates a prejudice that deflates his credibility in the listener’s mind. He suffers a double wrong — the original injury and the epistemic wrong of not being credited as a source of testimony about his own life. The structural point is that testimony does not merely succeed or fail piece by piece; it operates inside a credibility economy in which some speakers are systematically discounted and others systematically overweighted. Whichever side of the Hume/Reid dispute you take, testimonial injustice is a real phenomenon — for the reductionist, a discrimination in how the inferential checklist is applied; for the anti-reductionist, a corruption of the default itself. Fricker’s companion category, hermeneutical injustice, bears on shared conceptual resources rather than on the credibility of individual speakers.

5.5 Questions to Argue About

• Hume’s miracle argument in §X is reductionism applied to the hardest case. Does the argument require general reductionism about testimony, or is it consistent with Reid’s credulity principle plus a specific defeater for “miraculous” reports? Read Hume §X.13 (“the plain consequence is […] that no testimony is sufficient to establish a miracle”) and decide.
• Coady argues that reductionism is self-undermining: you cannot independently verify enough of the testimonial record to ground a general trust in testimony. Is this a fatal objection, or can a local reductionism (case-by-case checking) avoid it?
• Suppose Reid is right and we have a default propensity to believe what we are told. Does this principle apply equally to your parents, to a stranger asking for directions, to an anonymous Wikipedia editor, to an advertisement, and to a chatbot? If not, on what grounds do you draw the lines — and is that drawing-of-lines just reductionism with extra steps?
• Fricker’s testimonial injustice presupposes that there is a “right” amount of credibility owed to a speaker, against which under-crediting is a wrong. Where does that owed amount come from? Is it Hume’s evidence-proportional measure or Reid’s default-acceptance, and does Fricker’s argument work better on one of the two?

6 What is intuition — and can it count as knowledge?

You see someone’s face and immediately know they are angry. You sit down to a maths problem and — before you’ve done any calculation — you “feel” the answer is around 500. You’re at a dinner party and something about the other guest makes you uneasy, though you couldn’t say why. These are all called “intuitions” — though already the noun is doing too much work, since the three cases involve quite different cognitive processes. The chapter that follows treats them under one heading because they share the phenomenology of immediacy, but stay alert to the lump: Kant’s a priori, Klein’s expert pattern-recognition under feedback, and Kahneman’s System 1 are all called “intuition” and are not the same thing. Philosophers disagree about whether each of these is a cognitive achievement or a cognitive trap, and the answers come out differently by case.

6.1 Kant and A Priori Knowledge

Immanuel Kant made one of the most important distinctions in epistemology: between knowledge that comes from experience (a posteriori knowledge) and knowledge that is independent of experience (a priori knowledge).⁴⁸

“\(2 + 2 = 4\)” is a priori: you don’t need to go count things to know it. But Kant argued that some a priori knowledge is synthetic — it adds information about the world, not just unpacking definitions. The laws of Euclidean geometry were his central example. These, Kant thought, were conditions that the human mind imposes on experience, rather than reading off it.

The deeper point: Kant believed the mind is not a blank slate that receives impressions passively. It actively structures experience using categories (causation, substance) and forms of intuition (space and time). These are not learned — they are the conditions for any experience at all. They are built in.

Kant is not being mystical. He is making a specific philosophical claim: some knowledge does not need empirical justification because it constitutes the framework within which empirical justification operates. Whether Kant is right about the details is contested. Whether the category of a priori knowledge is real is also contested, though less widely: Quine’s “Two Dogmas of Empiricism” (1951) argues that even apparent a priori truths are merely the most entrenched beliefs in our web — beliefs we would give up last but could in principle give up.

6.2 Moral Intuitions: Rawls’ Reflective Equilibrium

In A Theory of Justice (1971), John Rawls described a method for moral reasoning he called reflective equilibrium (§9).⁴⁹ The idea: we start with our pre-theoretical moral intuitions — the strong sense that torturing children for fun is wrong, that people should not be punished for crimes they didn’t commit. These intuitions are data. We then construct general principles that accommodate as many of them as possible. Where the principles and intuitions conflict, we revise — sometimes the principle, sometimes the intuition.

“We can think of the original position as a device of representation.” — John Rawls, Political Liberalism, Lecture I (“Fundamental Ideas”)⁵⁰

Rawls is treating moral intuitions as evidence. Not infallible evidence — intuitions can be culturally biased, historically contingent, or the product of prejudice. But evidence nonetheless. This is different from saying intuitions are just feelings to be explained away by science.

The Trolley Problem (see Ethics unit) works precisely because our intuitions give incompatible verdicts: we feel pulling the lever is permissible, but pushing a fat man off a bridge to stop the trolley feels like murder, even though the arithmetic of consequences is identical. Intuitions are tracking something — but what?

6.3 Expert Intuition: Kahneman’s System 1

Daniel Kahneman (Thinking, Fast and Slow, 2011) distinguishes between:⁵¹

• System 1: fast, automatic, intuitive, unconscious processing. It recognises patterns instantly.
• System 2: slow, deliberate, analytical, conscious reasoning.

Kahneman’s point: System 1 is often right, but for specific reasons. A chess grandmaster who “sees” the right move after a glance is not guessing. She is pattern-matching against tens of thousands of hours of encoded experience. Naturalistic Decision Making research (Gary Klein) shows that expert firefighters, nurses, and pilots routinely make excellent rapid decisions that they cannot fully articulate.⁵²

But System 1 is also the source of cognitive biases: the availability heuristic, anchoring, the representativeness heuristic. Intuitions fail systematically in predictable ways — when situations are novel, when feedback is delayed, when patterns are statistically misleading.

Intuition is reliable when it is trained expert intuition in a regular domain with good feedback. It is unreliable in novel, high-stakes, or complex statistical domains. The question is: which kind of intuition do we have when we think we “just know” something in ethics, or philosophy, or everyday life?

6.4 Intuition in Mathematics and Science

Many mathematicians describe their work as guided by aesthetic intuition — a sense of which conjectures are worth pursuing, which approaches feel elegant. Henri Poincaré described working on a difficult problem, then boarding a bus, and at the exact moment his foot touched the step, the solution appearing fully formed:

“The ideas rose in crowds; I felt them collide until pairs interlocked, so to speak, making a stable combination.” — Henri Poincaré, Science and Method (1908), Part II, Chapter III (“Mathematical Discovery”)⁵³

Physicists describe similar experiences. Einstein said that the general theory of relativity emerged from a “thought experiment” — intuitive visualisation — before the mathematics was worked out. The mathematical proof came after the intuition.

If intuition can be a source of knowledge in mathematics — a domain supposedly defined by rigorous proof — then the relationship between intuition and justification is more complicated than the textbook picture suggests.

6.5 Embodied Knowing: Merleau-Ponty

The accounts above — Kant’s a priori, Rawls’ reflective equilibrium, Kahneman’s System 1 — all treat intuition as something that happens in the mind, even if quickly or unconsciously. Maurice Merleau-Ponty (Phenomenology of Perception, 1945) challenged this picture at its root: knowing is not primarily a mental act. It is a bodily achievement.⁵⁴

A skilled cyclist does not think about balance — their body maintains it. A violinist does not consciously calculate finger positions — their hands know where to go. Merleau-Ponty’s term is the body schema: a pre-reflective, non-propositional knowledge that the body carries, that cannot be adequately captured as a set of beliefs or rules. When a blind person navigates with a cane, the cane becomes an extension of their body schema — they feel the world through it directly, not by consciously inferring from the pressure in their hand.

This matters for epistemology: it suggests that much of our knowledge of the world is not propositional (not a matter of holding beliefs that are true or false) but practical and embodied — a kind of knowing that cannot be fully translated into the JTB framework. The question “is it justified?” doesn’t quite apply to the cyclist’s balance. Whether this counts as a deficiency in the cyclist’s knowledge or as a limit in the JTB framework is itself contested. Defenders of JTB will say procedural skill is simply not knowledge in the relevant sense — it is ability or competence, distinct from knowledge that; Merleau-Ponty’s heirs will reply that any framework which excludes the bulk of competent action from “knowledge” has shrunk the concept until it no longer fits the human case. Both responses are available; what is not available is treating the JTB-incompatibility as if it settled the question.

6.6 The Evolutionary Roots of Moral Intuition

Kahneman’s System 1 is rapid and pattern-based, but where do the patterns come from? Primatologist Frans de Waal (Primates and Philosophers, 2006; The Age of Empathy, 2009) argues that the roots of human moral intuition go deeper than culture or rational reflection — they are evolutionary:

Capuchin monkeys will refuse a reward (a cucumber slice) if they observe another capuchin receiving a better reward (a grape) for the same task. The refusal is dramatic: they throw the cucumber at the researcher. This is not calculated indignation. It is a visceral response to perceived unfairness — a functional sense of equity in a non-human primate.⁵⁵

Chimpanzees console distressed companions, share food, reconcile after conflicts, and punish free-riders in cooperative tasks. De Waal’s conclusion: the building blocks of human morality — empathy, fairness, reciprocity, conflict resolution — are not unique to humans. They evolved in social species because they were adaptive.

This has a consequence for how we understand moral intuitions — though it leaves entirely open whether it has any consequence for how we should trust them. They are not arbitrary cultural deposits, and they are not purely rational constructions. They are the elaboration, in a language-using, culturally complex species, of responses that evolution shaped over millions of years because social living required them. When you feel that pushing someone off a bridge to save five people is wrong in a way that pulling a lever is not, you are responding from a deep structure that predates culture. Whether that deep structure justifies the response, however — or merely explains it — is a separate question, and one that the descriptive evolutionary account cannot settle on its own. (To slide from “evolution shaped this response” to “this response is correct” is to commit the naturalistic fallacy: the descriptive premise does not entail the normative conclusion.)

What the evolutionary account does explain is why moral intuitions feel so compelling, and why philosophical theories that ignore them entirely tend to produce conclusions that nobody can actually live by. Whether such conclusions are nevertheless correct is a further matter.

6.7 Questions to Argue About

• Is intuition a form of implicit knowledge, or just a feeling that may or may not track truth?
• Rawls treats moral intuitions as evidence. But intuitions about who counts as a person, or who deserves rights, have been profoundly wrong historically. How do we know which intuitions to trust?
• Kahneman shows that expert intuition is more reliable than novice intuition. Does this mean intuition is ultimately reducible to pattern-matching from experience — or is there something else going on?
• Kant argues that some knowledge is a priori — built into the structure of the mind. Is this the same as intuition, or something different?
• Merleau-Ponty says the body “knows” how to ride a bike without the mind propositionally knowing. Is this the same kind of knowledge as knowing that Paris is the capital of France — or a genuinely different kind? Does the JTB framework apply to it?
• De Waal’s capuchin monkeys refuse unfair treatment. Does this show that a sense of fairness is “natural,” or does it just show that capuchins have learned what behaviour gets them rewards?

7 How does shared knowledge differ from personal knowledge?

By 1955, internal documents would later show, the major American tobacco companies privately knew that cigarettes caused cancer. The scientific community had settled the claim by the same date. For another thirty years, “tobacco causes cancer” did not function as common public knowledge in American life. The example forces a definitional question: does “shared knowledge” mean knowledge held in common among those competent to evaluate it (the narrow reading), or knowledge functioning as part of a community’s working understanding of itself (the broad reading)? The two readings give different verdicts on the tobacco case. With that flagged: how does personal knowledge become shared in either sense? What makes collective knowledge reliable? And who counts as a knower?

7.1 Popper’s Falsifiability and the Mark of Shared Knowledge

Karl Popper (The Logic of Scientific Discovery, 1934; Conjectures and Refutations, 1963) proposed that what distinguishes science from non-science is falsifiability: a claim is scientific if and only if it specifies what evidence would count against it. A theory that can explain any possible observation is not scientific — it is unfalsifiable.⁶⁰

Einstein’s general theory of relativity predicted that starlight would bend around the sun — a precise, quantitative, risky prediction. Eddington’s 1919 eclipse observations confirmed it.⁶¹ But crucially: the theory could have been disconfirmed. That is what makes it science.

Popper’s criterion is a criterion for shared knowledge: falsifiable claims are in principle checkable by anyone, anywhere, with the right equipment. They are public. They are repeatable. They are open to criticism.

By contrast: “Everything happens for a reason” or “God loves you” may be deeply meaningful as personal knowledge, but they are unfalsifiable — no observation could disconfirm them. This doesn’t make them false or worthless. It makes them a different kind of claim.

7.2 The Wikipedia Problem

Wikipedia is enormous and openly editable by anyone. This raises a genuine epistemological puzzle:

• A 2005 study in Nature found Wikipedia science articles contained on average 4 errors per article to Britannica’s 3 — closer than most expected, though not a clean Wikipedia victory.⁶²
• But Wikipedia is unreliable in specific ways: recent events, niche topics, and contested political subjects suffer from systemic bias and edit wars.
• The reliability of any Wikipedia article depends on how many expert eyes have reviewed it, how contested the topic is, and how resistant the community is to motivated editing.

The interesting question is not “is Wikipedia reliable?” but “what makes any source of shared knowledge reliable?” The answer has to do with processes: peer review, replication, adversarial testing, transparent methods, and institutional accountability.

7.3 Social Epistemology and Epistemic Injustice

Testimonial injustice — Miranda Fricker’s term for the wrong done when a hearer discounts a speaker’s report on the basis of identity prejudice — belongs to the epistemology of testimony.⁶³ What belongs here, in shared knowledge, is Fricker’s companion category: hermeneutical injustice, which is not about a particular speaker’s credibility but about the shared conceptual resources by which any speaker can articulate experience at all. Before the term “sexual harassment” entered common vocabulary, women who experienced it had difficulty even naming what was being done to them. The gap was not in their experience but in the shared interpretive apparatus — and the socially powerful had disproportionately shaped that apparatus. Hermeneutical injustice is therefore a defect in the collective knowledge-making infrastructure, not in any individual transaction.

The epistemological consequence: who counts as a knower, whose testimony is taken seriously, and which experiences have names available for them are not personal matters. They are political ones. Shared knowledge is shaped by power — and Fricker’s two categories give the two layers at which that shaping operates: case-by-case credibility and the underlying vocabulary itself.

7.4 When the Machinery Works: A Case Study in Successful Shared Knowledge

The failures are vivid; the successes are taken for granted. Here is a success.

In March 2004 and June 2005, the South Korean scientist Hwang Woo-suk published two papers in Science claiming to have produced first a cloned human blastocyst and then eleven patient-specific embryonic stem-cell lines through somatic cell nuclear transfer — a landmark result that would have revolutionised regenerative medicine.⁶⁴ The papers received enormous international attention; Hwang was treated as a national hero in South Korea. Within a year, everything had collapsed. Suspicions were raised first by junior members of his own team, who reported anomalies in the data — and the coercive sourcing of human eggs from female lab members — to the Korean public broadcaster MBC’s investigative programme PD Su-cheop, which aired its initial exposé in November 2005. Other researchers attempted replication and could not produce his results. Science commissioned an independent investigation; Seoul National University conducted its own. On 10 January 2006 the SNU committee concluded that all eleven cell lines were fabricated; the next day, Science unconditionally retracted both papers.⁶⁵ Hwang was subsequently convicted of embezzlement and bioethics violations.

What is notable here is not the fraud — fraud happens. What is notable is the speed and mechanism of correction in this case. The system that detected the fraud is the one Popper described: transparent methods (published data that could be scrutinised), replication attempts by independent researchers, adversarial peer review operating across institutional and national lines, and a formal retraction process. No authority figure declared the papers fraudulent; the community of practice identified the specific discrepancies, applied the falsification criterion, and revised the knowledge base. From the first published suspicions to formal retraction took approximately six months. Many fraud cases run slower: Schön in physics, Stapel in social psychology, and Macchiarini in surgery each took years from suspicion to retraction, and some fraud is presumably never detected at all.

The contrast with the tobacco case can be read in two ways. On one reading, Hwang shows the epistemic machinery working as it is supposed to, and tobacco shows what happens when motivated actors target the social infrastructure that machinery depends on. On a different reading, Hwang is a rare and unusually fast case (the journalists, the junior whistleblowers, the SNU committee all functioned simultaneously) — closer to an exhibit of best-case conditions than to a base rate against which tobacco is the failure mode. Both readings recover the same practical lesson: the conditions under which the machinery corrects (public data, independent replication, no institutional interest in protecting the result, professional norms strong enough to survive the retraction of a hero) are identifiable, sometimes present, and sometimes blocked.

7.5 When Shared Knowledge Breaks Down: A Case Study

Students now alive have lived through one of the most dramatic crises of shared knowledge since the Enlightenment. Consider two cases:

COVID-19 and scientific authority (2020–2022): Epidemiologists reached rapid consensus on a number of key facts — transmission routes, vaccine efficacy, the role of prior immunity. These were falsifiable claims subjected to enormous scrutiny. And yet a substantial subset of the public in high-income countries rejected some of these claims. The empirical literature on what exactly drove vaccine refusal is more mixed than the satisfying “trust collapse” story suggests: partisan identity, prior health-system experiences, social-network effects, and individualised risk perception all contributed, and the relative weights vary by country and cohort. The narrower phenomenon — that epistemically well-supported claims sometimes fail to be taken up by parts of the public toward whom relevant institutions had become an object of organised distrust — is real, even if “institutional trust collapsed” is too broad a description. What does Popper’s falsifiability criterion tell us about it? The claims were falsifiable; the mechanism for adjudicating them did exist. Shared knowledge in the broad sense (functioning as common public understanding) requires more than the claim meeting epistemic criteria — it requires functioning institutions to make and communicate the claim, and populations willing to engage with those institutions. Where these conditions weaken, the epistemic machinery keeps producing knowledge in the narrow sense without that knowledge becoming common in the broad one.

Climate science and manufactured doubt (1980s–present): Naomi Oreskes and Erik Conway (Merchants of Doubt, 2010) documented a deliberate strategy, used first by the tobacco industry and later by fossil fuel interests, of manufacturing apparent scientific controversy where genuine consensus existed. The strategy was not to produce better evidence — it was to create the impression of a debate, exploiting the journalistic norm of “balance” to give fringe views equal airtime with scientific consensus. This is an attack on the social conditions for shared knowledge, not on the evidence itself.

Falsifiability is necessary, not sufficient. Popper’s criterion tells us what distinguishes a good scientific claim from a bad one. It does not tell us how to defend shared knowledge when well-funded actors are motivated to corrupt the epistemic environment. Social epistemology — the study of how communities produce, evaluate, and transmit knowledge — becomes essential. The question “what makes knowledge shared?” turns out to require not just epistemology but political economy.

7.6 Standpoint Epistemology

[VERIFY: unanchored — suggest anchoring “strong objectivity” to Sandra Harding, “Rethinking Standpoint Epistemology: What is ‘Strong Objectivity’?” in The Centennial Review 36 (1992), 437–470, or her Whose Science? Whose Knowledge? (Cornell 1991), ch. 6; for the naturalist reply, Susan Haack, Manifesto of a Passionate Moderate (Chicago 1998), ch. 7 (“Knowledge and Propaganda”), and Philip Kitcher, Science, Truth, and Democracy (OUP 2001), ch. 6]

Sandra Harding and other feminist epistemologists developed standpoint theory: knowledge is always situated. Knowers occupy particular social positions (gender, class, race, history), and those positions can make certain things more visible or less visible.

The provocative claim: the standpoint of the marginalized can provide epistemic advantages in certain domains. This is not a consolation prize. A worker on a factory floor knows things about how the factory operates that the CEO does not — things the CEO cannot learn from the quarterly report. A patient knows things about their own illness that the doctor cannot access from the outside. An enslaved person understands the plantation economy in ways the plantation owner structurally cannot. This is not relativism — it doesn’t say all standpoints are equally valid for all questions. It says standpoint matters for knowledge, and that the standard model of knowledge production (the detached, universal observer) systematically misses what partial perspectives reveal.

The objection: does standpoint theory commit itself to relativism about truth? If knowledge depends on standpoint, do different standpoints generate incompatible “truths”? Harding’s response: strong objectivity requires taking standpoints seriously as variables, not eliminating the knower from the picture; pretending the knower has no position produces not objectivity but concealed partiality wearing objectivity’s clothes. Naturalist critics (Susan Haack, Philip Kitcher, broadly Quinean philosophers of science) reply that situatedness is one variable among many that ordinary scientific method already tracks and discounts, and that Harding treats one failure-mode (concealed partiality) as if it were the only alternative to her programme. The choice between Harding’s “strong objectivity” and the naturalist response is one of the live disputes in social epistemology, not a settled matter.

7.7 Questions to Argue About

• Popper says the mark of scientific (shared) knowledge is falsifiability. But is mathematics falsifiable? Is history? Are all forms of shared knowledge scientific?
• Wikipedia’s reliability depends on crowd-sourced editing and adversarial correction. Is this fundamentally different from how academic peer review works — or the same thing with different social mechanisms?
• Fricker argues that disbelieving testimony for prejudiced reasons is an epistemic wrong, not merely a moral one. Do you agree that there is a distinctively epistemic wrong here — or is this just a moral wrong that happens to involve knowledge?
• If a person’s social position gives them epistemically privileged access to certain truths, does that mean the social sciences should deliberately seek out marginalized perspectives? What are the limits of this idea?

8 What is a knower?

Knowledge requires a knower. But who — or what — is the knower? A disembodied thinking thing, as Descartes suggested? A bundle of impressions, as Hume argued? A socially situated embodied person? An artificial system? The question of what a knower is runs through epistemology, philosophy of mind, and personal identity — and it is less settled than you might expect.

8.1 Descartes’ Ego

After stripping everything away through radical doubt, Descartes arrives at the cogito: he exists as a res cogitans — a thinking thing. This is a substance, a subject. What kind of thing is it?

Descartes is a substance dualist: the mind is a non-physical substance, distinct from the body (res extensa). This creates the mind-body problem: how does an immaterial mind interact with a physical body? (Descartes’ answer — they interact in the pineal gland — is not taken seriously.) The problem of interaction has never been fully solved.

The Cartesian knower is:

• A unified self
• Transparent to itself (what it thinks, it knows it thinks)
• Essentially non-physical
• The same through time (numerical identity preserved by the thinking substance)

Each of these has been seriously challenged.

8.2 Hume’s Bundle Theory

Hume looked inward and reported what he found:

“For my part, when I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never can catch myself at any time without a perception, and can never observe any thing but the perception.” — David Hume, A Treatise of Human Nature, Book I, Part IV, Section VI⁶⁹

There is no self beyond the stream of perceptions. The self is a “bundle or collection of different perceptions, which succeed each other with inconceivable rapidity.” What we call the “self” is a convenient fiction — the impression of continuity overlaid on a flux.

If the knower is not a stable, unified thing, then who is accumulating knowledge over time? Who is the “I” that knows something today that “I” didn’t know yesterday?

8.3 Locke on Personal Identity and Memory

Consider Leonard Shelby, the protagonist of Christopher Nolan’s Memento (2000). Leonard suffers from anterograde amnesia — he cannot form new long-term memories. Every morning he wakes with no recollection of the previous day. He navigates the world through photographs, tattoos, and Polaroids with handwritten notes. He is, in some important sense, the same person from day to day — same body, same pre-injury memories, same name. But is he the same knower? Each day he must reconstruct his epistemic situation from external records he cannot remember making. Knowledge that ordinary knowers accumulate over time — who to trust, what has already been tried, what conclusions have been drawn — he must reinvent or read from a note. Leonard’s condition makes vivid what the memory theory of personal identity — developed below by John Locke — implies: the knower is constituted by the continuity of memory, and when memory breaks, the knower is in some sense fractured.

John Locke (An Essay Concerning Human Understanding, Book II, Chapter 27) gave a different answer: personal identity consists in continuity of consciousness, specifically memory.⁷⁰ You are the same person who was punished at school because you remember being punished. You are not the same person as Socrates because you remember nothing of being Socrates.

Derek Parfit (Reasons and Persons, 1984) pushed this to its limit with thought experiments:⁷¹

The teleporter case: You step into a teleporter. Your body is destroyed and an exact replica — atom-for-atom — is assembled on Mars with all your memories. Is the person on Mars you? If yes: what survived the destruction of your body? If no: when exactly did you cease to exist?

Fission: Suppose your brain is divided and each half transplanted into a different empty body. Both resulting people have memories of being you. Both are psychologically continuous with you. But they can’t both be you (they’re different people from each other). So neither is you? So you’ve ceased to exist even though nothing bad has happened to either resulting person?

Parfit’s conclusion: personal identity is not what matters. What matters — for ethics, for prudential reasoning — is psychological continuity. Whether that constitutes strict identity is a further, less important question.

8.4 The Situated Knower

Feminist epistemologists and philosophers of science argued that the Cartesian picture of the knower as a disembodied, universal reasoner is not only philosophically wrong but politically loaded. The argument is not that Descartes happened to be a French Catholic gentleman of the 17th century — that biographical fact, on its own, no more impeaches the cogito than Newton’s Anglicanism impeaches the Principia. The argument is that the very content of his picture — the knower as Nobody in Particular, suspended in pure reason — does covert epistemic work: it treats the assumptions characteristic of his social position (which questions are worth asking, which experiences count as standard, what counts as a neutral starting point) as the universal default, and dresses partiality as objectivity. The “view from nowhere” systematically ignores that all knowers are somewhere, embodied, culturally situated; what the pretence conceals tends to be exactly the assumptions the dominant standpoint never has to make explicit.

Donna Haraway (Situated Knowledges, 1988) argued for partial perspective: knowledge claims that acknowledge their situatedness are more honest and, in certain respects, more objective than claims that pretend to a “god’s-eye view” they cannot have.⁷²

Sandra Harding’s strong objectivity requires that the social position of the knower be included as a variable in the evaluation of knowledge, not suppressed.⁷³ A scientist’s class, gender, and cultural assumptions affect what hypotheses they consider, what they observe, and how they interpret results. Ignoring this is not objectivity — it is concealed partiality.

8.5 Can Machines Know?

Artificial intelligence has made the question urgent: what is it for anything to be a knower? Large language models can pass medical licensing exams, write legal briefs, and produce plausible philosophical essays. They can produce plausible philosophical essays about whether large language models can know things. Are they knowers?

The standard responses:

• No — they lack understanding (John Searle’s Chinese Room argument, 1980): a system that manipulates symbols according to rules doesn’t understand the symbols, any more than someone who follows rules for manipulating Chinese characters understands Chinese.⁷⁴ Searle’s Chinese Room has been replied to in nearly as many ways as there are philosophers, but the intuition it pumps remains powerful.
• Yes, or approaching it — functionalists argue that if a system behaves in all the ways a knower behaves, the label applies. What more could “knowing” require? If you say “but there’s nothing it’s like to be the system,” you are invoking subjective experience as the criterion — which immediately raises the question of how you verify that criterion in other humans.
• It’s the wrong question — the concept of “knowing” was developed for human beings in human social contexts. Applying it to machines may require revision of the concept rather than a verdict either way.

8.6 Questions to Argue About

• Hume says there is no self beyond the stream of perceptions. If true, what follows for the concept of a knower? Can there be knowledge without a stable knower?
• Locke grounds personal identity in memory. But memory is notoriously unreliable. Does this mean our sense of who we are is built on an unreliable foundation?
• Parfit argues personal identity is not what matters — psychological continuity is. Does this view change how you think about self-interest, or about why death is bad?
• The Cartesian knower is disembodied. But you don’t think in your brain alone — you think through conversation, through writing, through the tools you use (Andy Clark’s “extended mind” thesis).⁷⁵ What counts as part of the knower?
• Should a sufficiently sophisticated AI be considered a knower? What exactly does it lack?

8.7 What Grounds Logical Knowledge?

Two positions bracket the dispute about what grounds logical inference itself — the question Carroll’s Tortoise raised: you cannot use modus ponens to justify modus ponens. Quine (Two Dogmas of Empiricism, 1951) argues that even logical laws are revisable: they are not categorically certain, only the most entrenched beliefs we hold — beliefs we would give up last, but could in principle give up.⁷⁶Bonjour (In Defense of Pure Reason, 1998) argues that logical laws are known a priori through rational insight, not merely by entrenchment or habit.⁷⁷

The dispute is not peripheral. It determines whether epistemological claims about justification, knowledge, and what a knower is rest on a foundation that is itself secure, or whether that foundation is as revisable as everything else. If Quine is right, the logical structure of arguments in epistemology is not categorically more certain than the empirical claims those arguments evaluate. If Bonjour is right, there is a domain of a priori knowledge that grounds the rest.

9 Media

Novels, films, and artworks that illuminate the questions above:

• Plato, The Allegory of the Cave (in Republic, Book VII) — Not a novel, but the most powerful thought experiment in the history of epistemology. Prisoners in a cave mistake shadows for reality. One escapes and sees the sun. What is the relationship between education and liberation from illusion?
• Jorge Luis Borges, “Tlön, Uqbar, Orbis Tertius” (in Ficciones, 1944) — A secret society invents an encyclopedia of an imaginary world; the invented world begins to replace the real one. A meditation on reality, language, and collective knowledge-making.
• Philip K. Dick, Do Androids Dream of Electric Sheep? (1968) — What distinguishes genuine empathy from its simulation? What is the status of artificial minds as knowers? The basis for Blade Runner.
• Christopher Nolan, Memento (2000) — A man with anterograde amnesia (no new long-term memories) investigates his wife’s murder. Parfit’s thought experiments on personal identity and memory made visceral.
• The Wachowskis, The Matrix (1999) — Brain-in-a-vat scepticism as blockbuster film. Legitimately useful for discussing Descartes, despite being pulp cinema.
• Ingmar Bergman (dir.), Persona (1966) — A nurse and her mute patient gradually exchange identities. Bergman’s most concentrated treatment of personal identity as a fluid, performed, and partly fictional construction. The 80-minute running time is itself an argument: the disintegration of the self is shown faster than any theory could describe it.
• Kazuo Ishiguro, Never Let Me Go (2005) — Clone children raised for organ donation. Questions of what it means to be a knower, a person, a self — obliquely and devastatingly rendered.

10 Bibliography

11 Notes