---
title: "What is Knowledge?"
sort: 100
section-id: epistemology
description: The JTB analysis of knowledge, the Gettier problem, and the major responses to Gettier.
language: en
---

# What is Knowledge?

Epistemology — from the Greek *episteme* (knowledge) and *logos* (account or study) — is the branch of philosophy concerned with the nature, sources, and limits of knowledge. Its central question is deceptively simple: what is it to know something?

## The Traditional Analysis: Justified True Belief

The dominant account in Western philosophy from Plato through most of the twentieth century held that knowledge is *justified true belief* (hereafter JTB). To know that *p* is to (1) believe that *p*, (2) have *p* be true, and (3) be justified in believing that *p*.

Each condition plays a role. The truth condition rules out lucky coincidences: if I believe the train departs at 10am and it in fact departs at 10am, I do not know this if I formed the belief by guessing. The belief condition rules out propositions I accept without endorsing: I may act as if London is south of Edinburgh (it is not) without believing this, in which case I cannot be said to know it. The justification condition — the most philosophically contested of the three — distinguishes knowledge from mere true belief: if I believe, on no grounds whatsoever, that there is a spider behind the bookcase, and there happens to be a spider there, I do not thereby *know* there is a spider. Knowledge requires that one's belief be appropriately supported.

The JTB analysis has Platonic roots: in the *Meno*, Socrates distinguishes knowledge (*episteme*) from right opinion (*ortho doxa*) by the presence of an "account" that tethers the belief to its object. In the *Theaetetus*, Plato examines and ultimately rejects several definitions of knowledge, leaving the question famously open.

## The Gettier Problem

In a short, devastating paper published in 1963, Edmund Gettier showed that the JTB analysis is insufficient ^[Gettier, E., "Is Justified True Belief Knowledge?", *Analysis* 23, 1963, pp.121-123]. He produced two counterexamples — cases where an agent has a justified true belief but, intuitively, does not know.

The original cases are somewhat technical, but their structure can be illustrated as follows. Smith has good evidence that Jones will get the job, and that Jones has ten coins in his pocket. He infers: "The man who will get the job has ten coins in his pocket." In fact, Smith himself gets the job — and unbeknownst to him, he also has ten coins in his pocket. Smith's belief is true and justified. But he does not know it, because his justification for the belief is the evidence about Jones, not about himself. The truth of his belief is, in the relevant sense, a matter of luck.

Gettier cases share a common structure: the belief is true, and it is justified, but the justification and the truth are *accidentally* connected in a way that undermines knowledge. The epistemic luck that disqualifies knowledge is sometimes called *veritic luck* — the belief could easily have been false, even given the justification.

## Responses to Gettier

The philosophical literature on Gettier is vast ^[For surveys, see Shope, R., *The Analysis of Knowing*, Princeton UP, 1983; Ichikawa, J. and Steup, M., "The Analysis of Knowledge", *Stanford Encyclopedia of Philosophy*, 2018]. Several broad strategies have been pursued.

**The No-False-Lemmas Approach.** Some responses add a fourth condition: knowledge requires that the justification not pass through any false beliefs. In the Smith-Jones case, Smith's inference passes through the false belief that Jones will get the job. This approach handles many Gettier cases but fails against variants that generate knowledge through no false belief.

**Defeasibility Theories.** Lehrer and Paxson proposed that knowledge requires that one's justification not be *defeatable* by true information ^[Lehrer, K. and Paxson, T., "Knowledge: Undefeated Justified True Belief", *Journal of Philosophy* 66, 1969, pp.225-237]. If there is some truth that, were the agent to learn it, would undermine her justification, she does not know. This captures the intuition that Gettier cases involve misleading justification, but the correct formulation of the defeasibility condition has proven elusive.

**Reliabilism.** Alvin Goldman proposed replacing the traditional internalist justification condition with an externalist one: knowledge requires that the belief be produced by a *reliable belief-forming process* ^[Goldman, A., "What is Justified Belief?", in *Justification and Knowledge*, ed. Pappas, Reidel, 1979]. A reliable process is one that tends to produce true beliefs in the actual world. Perception, memory, and valid inference are typically reliable; guessing and wishful thinking are not. Reliabilism handles Gettier cases naturally: if a belief is produced by a reliable process, there is no epistemic luck.

**Safety and Sensitivity Conditions.** Sosa and Nozick proposed modal conditions on knowledge. Nozick's *tracking theory* required that the belief "tracks" the truth: if *p* were false, the agent would not believe *p* (the sensitivity condition); and if *p* were true, the agent would believe *p* (the adherence condition) ^[Nozick, R., *Philosophical Explanations*, Harvard UP, 1981, ch.3]. Sosa's *safety* condition required that the agent could not easily have been wrong ^[Sosa, E., "How to Defeat Opposition to Moore", *Philosophical Perspectives* 13, 1999].

**Knowledge First.** Timothy Williamson has argued that the traditional project of analysing knowledge in terms of more basic conditions is fundamentally misguided ^[Williamson, T., *Knowledge and Its Limits*, Oxford UP, 2000]. Knowledge, he argues, is a primitive mental state — not reducible to belief plus conditions. Rather than asking what conditions must supplement belief to yield knowledge, we should take knowledge as the starting point and explain belief and justification in terms of it. The slogan is "knowledge first."

## Internalism and Externalism

The Gettier debate brought into focus a broader dispute about the nature of epistemic justification. *Internalists* hold that the factors that determine whether a belief is justified must be *accessible* to the agent — available through reflection alone ^[Chisholm, R., *Theory of Knowledge*, Prentice-Hall, 1966]. On this view, two agents who are internally identical (same beliefs, same phenomenal states) must be equally justified, even if their environments differ dramatically.

*Externalists* deny this. Goldman's reliabilism is paradigmatically externalist: whether a belief is produced by a reliable process is a fact about the external world, not something the agent can determine by reflection. An agent might have a perfectly reliable belief-forming process that she has no way of knowing is reliable.

The internalism/externalism debate intersects with questions about scepticism (discussed in Chapter 5). Externalism offers a natural reply to sceptical scenarios — brain-in-a-vat believers may have reliably formed beliefs even in their abnormal environment — but faces the challenge of explaining the felt force of sceptical intuitions, which seem to appeal precisely to considerations accessible by reflection.

## Knowledge and Understanding

A growing body of work distinguishes *knowledge that* (propositional knowledge) from *knowledge how* (ability knowledge) and from *understanding*. Ryle's distinction between knowing-that and knowing-how influentially challenged the assumption that all knowledge is propositional ^[Ryle, G., *The Concept of Mind*, Hutchinson, 1949]. Understanding — grasping why something is the case, how the pieces fit together — seems to go beyond a collection of propositional beliefs and is increasingly seen as a distinct epistemic achievement worthy of investigation in its own right.

The question "What is knowledge?" turns out, as Plato suspected, to be genuinely difficult. The Gettier problem demonstrated that the most natural answer — justified true belief — is insufficient, and the subsequent fifty years of philosophy have not produced a consensus on what must replace it. But the failure to find a reductive analysis does not mean we have learned nothing. We have learned precisely why the question is hard, and that is progress.
