Overview – The Definition of Knowledge

The definition of knowledge is one of the oldest questions of philosophy. Plato’s answer, that knowledge is justified true belief, stood for thousands of years – until a 1963 philosophy paper challenged this definition.

Edmund Gettier described two scenarios – now known as Gettier cases – where an individual has a justified true belief that is not knowledge.

Since Gettier’s challenge to the justified true belief definition, various alternative accounts of knowledge have been proposed. The goal of these accounts is to define ‘knowledge’ in a way that rules out Gettier cases whilst still capturing all instances of what we consider to be knowledge.

A Level philosophy looks at 5 definitions of knowledge:

Justified True Belief

It’s important to first distinguish the kind of knowledge we’re discussing in A level philosophy. Broadly, there are three kinds of knowledge:

  • Ability: knowledge howe.g. “I know how to ride a bike”
  • Acquaintance: knowledge of – e.g. “I know Fred well”
  • Propositional: knowledge that – e.g. “I know that London is the capital of England”

When we talk about the definition of knowledge, we are talking about the definition of propositional knowledge specifically.

The tripartite definition

In Theaetetus, Plato argues that for something to be knowledge it must be:

  • Justified
  • True
  • Belief

This is known as the tripartite definition of knowledge.

Necessary and sufficient conditions

Each of the three conditions above are necessary for knowledge.

For example, you can’t know something if it isn’t true. If someone said, “I know that the moon is made of green cheese” you wouldn’t consider that knowledge because it isn’t true.

Similarly, you can’t know something you don’t believe.

And finally, justification. Suppose you flip a coin and beforehand your friend says “I know it’s going to land on heads”.

How can they know this? Whether the coin lands on heads or tails is random – there’s no way you can realistically know beforehand which side it will land on. This doesn’t count as knowledge because it is not properly justified – even if he does get it right, it’s just a lucky guess, not knowledge.

Together, these necessary conditions (justified, true, and belief) are said to be jointly sufficient. This means they capture every instance of knowledge whilst not capturing anything that isn’t knowledge. This latter part is what Gettier cases challenge.

Problem: Gettier cases

Gettier’s paper describes two scenarios where an individual has a justified true belief that is not knowledge. One of Gettier’s scenarios is:

  • Smith and Jones are interviewing for the same job
  • Smith hears the interviewer say “I’m going to give Jones the job”
  • Smith also sees Jones count 10 coins from his pocket
  • Smith thus forms the belief that “the man who will get the job has 10 coins in his pocket”
  • But Smith gets the job, not Jones
  • And, by coincidence, Smith also has 10 coins in his pocket

Smith’s belief “the man who will get the job has 10 coins in his pocket” is:

  • Justified: he hears the interviewer say Jones will get the job and he sees that Jones has 10 coins in his pocket
  • True: the man who gets the job (Smith) does indeed have 10 coins in his pocket

But despite being a justified true belief, we do not want to say that Smith’s belief counts as knowledge because it’s just luck that led to him being correct.

This shows that the tripartite definition of knowledge is not sufficient.

Alternative definitions of knowledge

Gettier cases are a devastating problem for the tripartite definition of knowledge.

In response, philosophers have tried to come up with new definitions of knowledge that avoid Gettier cases.

Generally, these new definitions seek to refine the justification condition of the tripartite definition. True and belief remain unchanged.

JTB + no false lemmas

The no false lemmas definition of knowledge aims to strengthen the justification condition of the tripartite definition.

It says that James has knowledge of P if:

  • P is true
  • James believes that P
  • James’s belief is justified
  • James did not infer that P from anything false

So, basically, it adds an extra condition to the tripartite definition. It says knowledge is justified true belief + that is not inferred from anything false (a false lemma).

This avoids the problems of Gettier cases because Smith’s belief “the man who will get the job has 10 coins in his pocket” is inferred from the false lemma “Jones will get the job”.


  • The tripartite definition says Smith’s belief is knowledge, even though it isn’t
  • The no false lemmas response says Smith’s belief is not knowledge, which is correct.

So, in this instance, the no false lemmas definition appears to be a more accurate account of knowledge than the tripartite view. It avoids saying Gettier cases count as knowledge.

Problem: fake barn county

However, the no false lemmas definition of knowledge faces a similar problem: the fake barn county situation:

Justified True Belief and Knowledge Venn Diagram
Venn diagram highlighting the problems with the JTB and JTB+N definitions. JTB+N is an improvement on JTB, but the fake barn county example shows that this definition still includes cases that are not knowledge.
  • In ‘fake barn county’, the locals create fake barns that look identical to real barns
  • Henry is driving through fake barn county, but he doesn’t know the locals do this
  • Henry often thinks “there’s a barn” when he looks at the fake barns
    • These beliefs are not knowledge, because they are not true – the barns are fake
  • However, on one occasion Henry looks at the one real barn and thinks “there’s a barn”
    • This time the belief is true
    • It’s also justified by his visual perception of the barn
    • And it’s not inferred from anything false.

According to the no false lemmas definition, Henry’s belief is knowledge.

But this shows that the no false lemmas definition must be false. Henry’s belief is clearly not knowledge – he’s just lucky in this instance.


Reliabilism says James knows that P if:

  • P is true
  • James believes that P
  • James’s belief that P is caused by a reliable method

A reliable method is one that produces a high percentage of true beliefs.

So, if you have good eyesight, it’s likely that your eyesight would constitute a reliable method of forming true beliefs. If you have an accurate memory, it’s likely your memory would also be a reliable method for forming true beliefs.

But if you form a belief through an unreliable method – for example by simply guessing – then it would not count as knowledge even if the resultant belief is true.

Children and Animals

An advantage of reliabilism is that it allows for young children and animals to have knowledge. Typically, we attribute knowledge to young children and animals. For example, it seems perfectly sensible to say that a seagull knows where to find food or that a baby knows when its mother is speaking.

However, pretty much all the other definitions of knowledge considered here imply that animals and young children can not have knowledge. For example, a seagull or a baby can’t justify its beliefs and so justified true belief rules out seagulls and young babies from having knowledge. Similarly, if virtue epistemology is the correct definition, it is hard to see how a seagull or a newly born baby could possess intellectual virtues of care about forming true beliefs and thus possess knowledge.

However, both young children and animals are capable of forming beliefs via reliable processes, e.g. their eyesight, and so according to reliabilism are capable of possessing knowledge.

Problem: fake barn county

You can argue against reliabilism using the same fake barn county argument above: Henry’s belief that “there’s a barn” is caused by a reliable cognitive process – his visual perception. Even though it has let him down in this particular case, it’s normally a reliable method of forming true beliefs.

Thus, reliabilism would (incorrectly) say that Henry knows “there’s a barn” even though his belief is just lucky.

Virtue epistemology

One virtue epistemology definition of knowledge would be:

  • P is true
  • James believes that P
  • James’s true belief that P is a result of James exercising his intellectual virtues

Intellectual virtues are traits that lead you to reliably form true beliefs. For example, good memory, accurate vision, and the ability to think rationally could be considered intellectual virtues.

But where virtue epistemology differs from reliabilism is that it specifies the true belief must be a direct result of exercising intellectual virtues. It’s not enough for the method used to form the belief to be generally reliable (as reliabilism claims) because that doesn’t rule out lucky cases such as fake barn county.

The philosopher Ernest Sosa illustrates virtue epistemology with the following archery analogy: A virtuous shot in archery has the following three properties:

  • Accurate, i.e. it hits the target
  • Adroit, i.e. the archer is skillful and shoots the arrow well
  • Apt, i.e. the arrow hit the target because it was shot well

This last condition – aptness – is the difference between reliabilism and virtue epistemology. It’s not enough for a belief to be true and for the believer to be intellectually virtuous. For something to qualify as knowledge, the belief must be true as a consequence of the believer exercising their intellectual virtues.

So, virtue epistemology could (correctly) say Henry’s belief that “there’s a barn” in fake barn county would not qualify as knowledge – despite being true and formed by a reliable method – because it is not apt. Yes, Henry’s belief is accurate (i.e. true) and adroit (i.e. Henry has good eyesight etc.), but he only formed the true belief as a result of luck, not because he used his intellectual virtues.

Linda Zagzebski: What is Knowledge?

Linda Zagzebski is another advocate of virtue epistemology. On Zagzebski’s analysis of knowledge, James knows that P if:

  • James believes that P
  • James’s belief that P is a result of him exercising his intellectual virtues

Note that Zagzebski does not include the condition ‘true’ in her definition of knowledge. She does, elsewhere in her work, describe knowledge as “cognitive contact with reality” however.

In Zagzebski’s analysis of knowledge, the truth of the belief is kind of implied by the idea of intellectual virtues.

Firstly, virtues motivate us to pursue what is good (see Aristotle’s virtue theory in moral philosophy). Good knowledge is also true, and so another intellectual virtue would be something like: caring about having true beliefs.

Secondly, virtues enable us to achieve our goals (in the same way a virtuous i.e. good knife enables you to cut) and so intellectual virtues would enable you to reliably form true beliefs.

Problem: children and animals

As mentioned in more detail in the reliabilism section above, a potential criticism of virtue epistemology is that it appears to rule out the possibility of young children or babies possessing knowledge, despite the fact that they arguably can know many things.


Infallibilism argues that for a belief to count as knowledge, it must be true and justified in such a way as to make it certain.

So, even though Smith has good reasons for his beliefs in the Gettier case, they’re not good enough to provide certainty. Certainty, to philosophers like Descartes, means the impossibility of doubt.

In the Gettier case, Smith might have misheard the interviewer say he was going to give Jones the job. Or, even more extreme, Smith might be a brain in a vat and Jones may not even exist! Either of these scenarios – however unlikely – raise the possibility of doubt.

Problem: too strict

So, infallibilism correctly says Smith’s belief in the Gettier case does not count as knowledge.

But it also says pretty much everything fails to qualify as knowledge!

“I know that water boils at 100°c” – can this be doubted? Of course it can! Your science teachers might have been lying to you, you might have misread your thermometer, you might be a brain in a vat and there’s no such thing as water!

Pretty much any belief can be doubted, as Descartes demonstrates in his three waves of doubt.

So, whereas Gettier cases show the tripartite definition to set the bar too low for knowledge, infallibilism sets the bar way too high – barely anything can be known!

Knowledge from Perception>>>