Knowing is having a true opinion and being able to give an account of it

The definition of knowledge as justified true belief; the Theaetetus

Plato c. 428 BC · c. 348 BCBeyond appearances, the eternal Forms.

What it claims

Knowledge is true opinion accompanied by reason (logos): what one can give an account of is an object of knowledge, the rest is not.

Why

Neither perception nor true opinion is enough: a judge can rule rightly, persuaded by witnesses, without knowing the facts.

The same idea through history

Plato385 BCIn the Meno (97e-98a), Socrates already explains that a true opinion is not yet knowledge: it is like a statue that runs away until it is tied down by reasoning about its cause.

Plato369 BCIn the Theaetetus, three answers are examined: knowledge is sensation, knowledge is true opinion, knowledge is true opinion accompanied by reason (logos). The third is what is now called "justified true belief".

Alfred Jules Ayer1956In the 20th century Ayer takes up the formula: to know is to have a true belief, to be sure of it, and to have the right to be sure.

Roderick Chisholm1957Chisholm puts it this way: to know is to accept a true proposition for which one has adequate evidence. Gettier cites Ayer and Chisholm as recent defenders of the definition he attacks in 1963.

In short

"Knowing is having a true opinion and being able to give an account of it" is an argument by Plato (Plato, Theaetetus, 201c-d (c. 369 BC), 385 BC) on the question "Can we be certain of anything?" (answer: yes, through reason). Its claim: Knowledge is true opinion accompanied by reason (logos): what one can give an account of is an object of knowledge, the rest is not. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 2 premises. AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Plato, Theaetetus, 201c-d (c. 369 BC)

Valid

What is knowing? In the Theaetetus, the young Theaetetus offers Socrates three answers. First: knowing is sensing, perceiving (this is also the thesis of Protagoras, "man is the measure of all things"). Socrates refutes it. Next: knowing is having a true opinion, but a court can judge rightly by being merely persuaded, without knowing. The third remains: knowing is having a true opinion accompanied by reason (logos), that is, being able to explain why it is true. It is the most famous definition of knowledge, and it stayed standard for more than two thousand years. But Plato does not say it is the right one: the dialogue ends without a conclusion. We present here the argument that leads to this definition as the dialogue brings it out, and we also give its difficulties.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.

Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where knowledge is neither sensation nor true opinion, without being true opinion accompanied by reason (for example, a fourth candidate that nobody has yet proposed).

The argument is valid with P3: two candidates out of three are set aside, the third remains. Without P3 it is no longer valid: one could have ruled out sensation and true opinion without "true opinion plus reason" being the right definition. That is exactly Plato's problem: Socrates then examines three senses of the word logos and rejects them all (201d-210a), then leaves the dialogue without an answer. The strength of the argument lies elsewhere: even if incomplete, the distinction between a true opinion held by luck and knowledge resting on reasons became the basis of the theory of knowledge.

Analogy. A pupil lands on the right answer to a maths problem by guessing. He has a true opinion, but we do not say he knows. Another pupil finds the same answer and can show how he got there: of him we say he knows.

Objections

Each objection is a conversation: click to see who replies and where the debate stands.

Theaetetus and Protagoras 369 BC · attacks premise 1

Knowing is sensing: whoever knows something perceives what he knows. And if, as Protagoras says, man is the measure of all things, each thing is for me as it appears to me, for you as it appears to you: there is nothing more to know than what one perceives.

Platon replies

Socrates (in Plato) Socrates' reply (186d-e)

Knowledge is not in the sensations but in the reasoning about them: it is by reasoning that one can grasp being and truth, which sensation cannot do. Sensation and knowledge are therefore not the same thing.

Socrates (in Plato) Socrates turns Protagoras' thesis against itself (171a-b)

If Protagoras grants that the opinion of those who think he is wrong is true, he admits that his own opinion is false.

The debate goes on (our summary) Scholars discuss how strong this refutation is: Protagoras could answer that his thesis is true only for him. In the dialogue Theaetetus is convinced, and the text is written so that we are too.

Socrates (in Plato) 369 BC · attacks the conclusion

If we ask what knowledge is, answering that it is true opinion accompanied by knowledge of what distinguishes it from everything else is a silly answer: we use the word "knowledge" to define knowledge. More generally, Socrates examines three senses of "logos" (expressing one's thought, listing the elements, naming the distinguishing mark) and none holds.

Platon replies

End of the dialogue (210a-b)

Socrates concludes that neither sensation, nor true opinion, nor true opinion accompanied by an explanation is knowledge. The dialogue stops there, without a definition: it has only got rid of false answers.

The debate goes on (our summary) This is why Plato cannot be presented as the defender of the definition: a character, Theaetetus, says he heard it "in a dream", and Socrates tests it. The history of philosophy has nevertheless kept the formula and forgotten the failure.

Edmund Gettier 1963 · attacks the conclusion

One can have a true opinion, be justified in holding it, and yet not know. Example (case I): Smith has good reasons to believe 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". But Smith gets the job, and he happens to have ten coins in his pocket, without knowing it. His belief is true and justified, but we would not say he knows. The three conditions (true, believed, justified) are therefore not enough.

Platon replies

Alvin Goldman The causal condition (1967)

Goldman proposes adding a condition: for someone to know, the fact that makes his belief true must be connected in the right way, through a causal chain, to that belief. In Gettier's cases, Smith's belief is not caused by the fact that makes it true (the coins in his own pocket).

Alvin Goldman The fake barn case (1976)

Henry drives through the countryside and looks at a real barn. His belief "that is a barn" is true and caused by the barn. But the region is full of cardboard fake barns he cannot tell from a real one. It seems he does not know, despite the causal link: the causal condition is therefore not enough either.

See the rest of the debate (2)

Alvin Goldman Reliabilism (1979)

"Reason" is replaced by an external criterion: a true belief is knowledge if it is produced by a reliable process, that is, one that mostly yields true beliefs. The subject no longer needs to be able to explain his belief: this gives up Plato's logos.

Linda Zagzebski 1994

As long as a theory leaves a gap between what justifies or makes a belief reliable and its truth, one can always build a Gettier-style case: add a stroke of luck that makes the belief true anyway. The problem would thus be unavoidable for almost all definitions.

The debate goes on (our summary) Sixty years later there is no definition accepted by all. Many philosophers conclude that knowledge may be basic and cannot be broken down into ingredients (Timothy Williamson's thesis, 2000), others are still looking for the fourth condition. Gettier himself published almost nothing else.