Knowledge

• 1. Introduction

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  Knowledge has a definition philosophers have been arguing about for centuries, and the argument is not close to settled. A person drives down a country road lined with barn facades. Only one of them is a real barn. By a lucky coincidence, they stop in front of the only genuine one and conclude, correctly, that they are looking at a barn. Their belief is true. They have reasons for it. And yet many philosophers insist this person does not actually know they are facing a real barn, because they could not have told the difference. This is the kind of puzzle that sits at the center of knowledge. What separates knowing something from merely being right by accident? Where does knowledge come from, and how far can it reach? Who decides whether belief in God, or in a moral rule, counts as knowing at all? The word itself traces back to the 12th-century Old English cnawan, but the questions behind it stretch much further. What follows is an attempt to take that ordinary word apart and see what holds it together.

• 2. Justified True Belief

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  Justified true belief is the definition most often handed down in classrooms, and it names three features at once. A claim must be believed, it must be true, and the belief must be justified. Truth does real work here. One can believe something false, but one cannot know something false. Belief is required too, which is why the everyday outburst "I do not believe it, I know it" is usually just a way of stressing confidence, not denying that a belief is involved.

  Justification is the part that draws fire. It is included because some true beliefs plainly are not knowledge. A person who guesses a coin will land heads, and happens to be right, does not thereby know it. Superstition, lucky guesses, and faulty reasoning can all land on the truth without amounting to knowledge. Some philosophers say a belief is justified when it rests on evidence such as experience, memory, or other beliefs. Others say it is justified when produced by reliable processes like sensory perception or logical reasoning.

  Edmund Gettier broke this tidy picture in the 20th century. The epistemologist assembled counterexamples in which a belief is true and justified yet still fails to be knowledge, the barn-facade case among the most famous of their kind. The failure traces to epistemic luck. The justification is real, but it has nothing to do with why the belief happens to be true. In response, some philosophers dropped justification altogether and turned to reliability or the manifestation of cognitive virtues. Others kept justification and added conditions, like requiring that the belief not depend on any false beliefs, or that no defeaters be present, or that the person would not hold the belief if it were false. Pragmatists shifted the focus to inquiry and habits of action. There is still very little consensus about which fix is correct.

• 3. The Many Kinds

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  "I know that Dave is at home" and "she knows how to swim" use the same verb for very different things. The first is propositional knowledge, also called declarative or descriptive knowledge, the paradigmatic type in analytic philosophy. It relates a knower to a proposition, often through a that-clause, as in "Akari knows that kangaroos hop." It spans specific facts, such as that the atomic mass of gold is 196.97 u, and generalities, such as that the leaves of some trees change color in autumn. Because it leans on mental representations, the capacity for it is often held to belong only to fairly sophisticated creatures.

  Knowledge-how needs no such representation. To know how to ride a bicycle or how to swim is to possess a practical ability, and some forms of it are common in the animal kingdom. An ant knows how to walk despite presumably lacking a mind able to represent the matching proposition. Knowledge by acquaintance is a third form, built from direct experiential contact. Eating chocolate acquaints a person with its taste; visiting Lake Taupō yields acquaintance with Lake Taupō. Bertrand Russell, who introduced the concept, held that acquaintance is more basic than propositional knowledge, since understanding a proposition requires acquaintance with its constituents.

  A priori and a posteriori knowledge split along the role of experience. Seeing that it rains or hearing a baby cry yields a posteriori, empirical knowledge. That 2 + 2 = 4 is traditionally taken as a priori, since no empirical investigation confirms it. Explaining how a priori knowledge is even possible is hard, and some empiricists deny it exists. Plato answered that the soul already holds the knowledge and only needs to recollect it. Descartes treated such knowledge as innate, present in every human mind. Beyond these well-worn pairs lie self-knowledge of one's own mental states, metaknowledge or knowledge about knowledge, the gap between explicit and tacit knowledge, and the Eastern distinction between higher and lower knowledge, named para vidya and apara vidya in Hinduism and the two truths doctrine in Buddhism.

• 4. Where It Comes From

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  Perception is usually named the most important source of empirical knowledge, and it is busier than it looks. Knowing a baby is sleeping counts as observational knowledge if a perception of the snoring baby caused it, but not if the fact arrived through a phone call with one's spouse. Perception runs across vision, sound, touch, smell, and taste, each tied to different physical stimuli, and it is an active process that selects, organizes, and interprets signals. That activity sometimes misfires into illusions like the Müller-Lyer illusion and the Ponzo illusion.

  Introspection turns the same lens inward, onto internal mental states rather than external objects. A long-standing view holds it infallible, on the grounds that one cannot be mistaken about whether one is in pain. Critics push back, suggesting a person might mistake an unpleasant itch for a pain, or confuse a slight ellipse for a circle. Memory differs from both, because it depends on previous experiences rather than standing on its own; it is generally reliable yet can deceive when the original experience was faulty or the memory has degraded.

  Inference builds new knowledge from old. Seeing a Czech stamp on a postcard can yield the inferential knowledge that a friend is visiting the Czech Republic. Some rationalists add rational intuition, arguing that beliefs like 2 + 2 = 4 are justified by pure reason alone. Testimony stands apart from all of these, tied to no single faculty. One person can come to know a fact because another talks about it, through speech, a letter, a newspaper, or a blog. The problem of testimony asks when this transfer actually produces knowledge, and the common answer points to the reliability of whoever is speaking.

• 5. The Regress Problem

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  "Ford cars are cheaper than BMWs" looks like an easy belief to hold until someone asks why. The believer might answer that they heard it from a reliable source. But that justification leans on the assumption that the source is reliable, which can be challenged in turn, and so can each reason offered after it. The epistemic status at every step depends on the status of the step before, which threatens an infinite regress. Theories of the structure of knowledge are attempts to halt that slide.

  Foundationalism stops the regress by declaring some reasons basic. These basic reasons hold their epistemic status independently of others and form the endpoint. Some foundationalists say perception supplies them; others point to self-evident truths, like knowledge of one's own existence. The view is not universally accepted. One objection asks why some reasons should count as basic and not others, suggesting their status secretly depends on further reasons. Another, drawn from hermeneutics, argues that all understanding is circular and requires interpretation, so knowledge needs no secure foundation at all.

  Coherentism rejects the basic versus non-basic split entirely. It holds that a finite set of beliefs mutually support and justify one another, forming a web rather than resting on a few privileged foundations. The difficulty is avoiding circular reasoning, since mutual support alone is not a good reason to accept both beliefs at once, and distinct coherent sets can compete with no clear way to choose between them. Infinitism takes the boldest route, embracing an infinite chain of reasons in which each depends on the next. Its problem is plain enough: the human mind is limited, which raises the question of whether infinitism leaves any human knowledge possible.

• 6. The Edges Of Knowing

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  Caesar's breakfast on the day he was assassinated may be unknowable to anyone alive now, because no significant traces of it survive, though it was perfectly knowable to him and some contemporaries. This marks one limit of knowledge: missing access to information. Others come from the limits of human cognitive faculties, since some truths are simply too complex for the human mind to conceive. A stranger limit arrives by way of logical paradox. There are ideas that will never occur to anyone, and they cannot be known, because if a person knew one, it would have occurred at least to them.

  Whole fields draw their own boundaries. Religious skepticism denies that beliefs about God amount to knowledge. Moral skepticism includes the claim that one cannot know what is morally good. Immanuel Kant proposed an influential limit on metaphysics: knowledge reaches only the field of appearances and never the things in themselves, so questions like whether the world has a beginning cannot be answered. The empirical sciences have their own walls, including the uncertainty principle, which forbids knowing the exact position and momentum of a particle at once, and chaotic systems whose sensitivity to initial conditions produces the butterfly effect.

  Radical or global skepticism pushes hardest, holding that knowledge is impossible. The dream argument claims perceptual experience is no source of knowledge, since a person could be dreaming without knowing it and cannot tell dream from waking perception. It rests on underdetermination, the situation where evidence cannot rationally decide between competing theories. A sharp reply notes that radical skepticism is self-contradictory, since denying that knowledge exists is itself a knowledge-claim. Very few philosophers have defended the position outright, yet it has shaped the field as the challenge every theory feels obliged to overcome. Fallibilists take the gentler line: error can never be fully excluded, so even the best-researched theories might be wrong, though most fallibilists still hold that knowledge exists.

• 7. What It Is Worth

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  A cup of coffee from a reliable machine tastes no better than an equally good cup from an unreliable one. Philosopher Linda Zagzebski uses that analogy to press a hard question: why is knowledge worth more than a true belief that merely happens to be reliable? Knowledge can be useful, carrying instrumental value when it helps a person pass an exam or pick the fastest horse. But not all of it is useful. Counting the grains of sand on a beach has no instrumental value, and in rare cases knowledge can even harm, as when a true belief about the dangers of a ravine stops a person from making a life-saving jump.

  Intrinsic value is the other side. Some knowledge may be good in itself even without practical benefit, and philosopher Duncan Pritchard links this to forms of knowledge connected with wisdom. Whether all knowledge has intrinsic value is disputed, including trivial facts like whether the biggest apple tree had an even number of leaves yesterday morning. One argument holds that having no belief is a neutral state and knowledge always beats it, if only slightly.

  Plato's Meno frames the oldest version of the puzzle. Both knowledge and true belief can guide action equally well, so it seems they share the same value when, say, finding the way to Larissa. Plato answers that knowledge is better because it is more stable. Reliabilism, which defines knowledge as reliably formed true belief, struggles to explain the added worth, which is why the coffee analogy is sometimes used against it. Virtue epistemology offers a different solution, treating knowledge as the manifestation of cognitive virtues and locating its extra value in that cognitive success. The stakes are not only theoretical. Acquiring knowledge costs time, energy, and resources, so governments weigh which research to fund, businesses weigh whether the payoff justifies the cost, and educators weigh what to pass on to students.

• 8. Knowing Across Cultures

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  Around 3000 BCE, major civilizations rose in Mesopotamia, Egypt, India, and China, and with them came the invention of writing. Writing let knowledge be stored and shared without the limits of imperfect human memory, and the first developments in mathematics, astronomy, and medicine followed. The ancient Greeks formalized and expanded these fields starting in the 6th century BCE. In the medieval period, religious institutions like the Catholic Church shaped intellectual life, Jewish communities founded yeshivas, and madrasa schools in the Muslim world focused on Islamic law and philosophy. The Islamic Golden Age, from the 8th to 13th centuries, preserved and refined ancient learning and produced centers like Al-Qarawiyyin University in Morocco, Al-Azhar University in Egypt, and the House of Wisdom in Iraq. The printing press, invented in the 15th century, raised literacy and set the stage for the Scientific Revolution.

  The scientific method is now treated as the model process for gaining empirical knowledge. It begins with observation and data collection, moves to a hypothesis that explains the observations, tests that hypothesis with a controlled experiment, and interprets the results to judge whether they confirm or disconfirm it. Its strength is that the evidence is public, reliable, and replicable, so others can repeat the work. The natural sciences lean on quantitative methods and precise numerical measurement, while the social sciences give more weight to qualitative methods like interviews, focus groups, and case studies. Thomas Kuhn challenged the picture of steady progress, arguing that scientific revolutions bring paradigm shifts incommensurable with what came before. Scientism, the privileging of science above all other inquiry, drew criticism from philosophers such as Hans-Georg Gadamer and Paul Feyerabend, who found its insistence on one method too rigid.

  Religions place knowledge at their center, often in tension with skepticism. Asked how he would justify his disbelief in God before judgment, Bertrand Russell replied, "Not enough evidence, God! Not enough evidence." Kant struck a different balance, writing that he "had to deny knowledge in order to make room for faith." In the Jewish and Christian traditions, Adam and Eve ate from the tree of knowledge of good and evil and were expelled from the Garden of Eden. In Islam, al-ʿAlīm, "the Knowing," is one of the 99 names of God. Buddhism calls liberating knowledge vijjā, set against avijjā or ignorance, the root of all suffering, while Hinduism's jñāna yoga, the path of knowledge, seeks oneness with Brahman. The sociologist's lens adds yet another angle: Karl Marx tied a society's dominant ideology to its socioeconomic conditions, and Michel Foucault examined how knowledge controls people through what he called biopower in fields like psychiatry, medicine, and the penal system.