I have been told that knowledge is usually analyzed as being justified true belief (although this conception has been criticized - namely after Gettier published his famous article - it seems to be widely accepted, at least as a good approximation of what knowledge really is). However, I don't see why the truth condition is necessary. Indeed, by definition, we can only evaluate the truth value of a proposition by making appeal to our justification for this proposition. Moreover, believing some proposition is believing this proposition to be true. When I say "I know x", it seems therefore to me that I don't say anything else than "I believe x and I am justified in believing x", with the truth condition of the tripartite definition of knowledge encompassed by the belief and justification conditions. Can someone explain me then why knowledge is not defined as just "justified belief" ? Many thanks in advance.

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    Because the widespread intuition is that knowledge, like truth, is about how things are, not just about what we can evaluate. If we believe something to be true, and it is not, then we simply do not know enough about it, see truth condition.
    – Conifold
    Aug 3, 2020 at 17:53
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    This said, Hazlett recently suggested that in the ordinary talk the word "know" does not match its philosophical meaning, and knowledge in colloquial sense can be false, see The Myth of Factive Verbs:"The concept of knowledge that epistemologists have been interested in since the Meno is a factive concept (in the sense that nothing false can be known). But, if I‘m right, the concept of knowledge that serves as the meaning of 'knows' in ordinary talk isn‘t.".
    – Conifold
    Aug 4, 2020 at 5:25
  • Most issues in reality and everyday life is that people always conflate knowledge with seemingly justified sourced information abound everywhere believed by incautious people, that's why Bacon's maxim "Knowledge is Power" is no longer appreciated for quite a long time... Oct 11, 2021 at 2:53

5 Answers 5


Truth is required because you cannot know falsehoods. If Bob is justified in believing that his package was lost in the mail, because it's been 2 months already and it hasn't arrived, but the package is in fact now waiting outside his door, would you say that he knows that his package was lost? I think not. Bob may think that he knows that the package was lost, but he is mistaken.

If you have justified but false belief that p, you may think you know p, but you don't. The truth condition filters out such cases as not genuine instances of knowledge

Here's another way to see this. The sentence "Alice believes that today is Sunday, and Bob believes that today is Monday" can be true. But "Alice knows that today is Sunday and Bob knows that today is Monday" seems contradictory. That's because "S knows that p" entails that p is true. So it's not possible for someone to know p, and for someone else to know not-p.

  • In the situation you describe, I would not say that Bob knows that the package was lost. However, this situation seems to me very artificial because in the real world, truths are not given the way you did. There is no book containing all the objective truths about the world (like "the package is in fact now waiting outside Bob's door", ...). So in real life when we use sentences like "I know that p", we do not deal with the truth condition. Similarly, when I say "Bob knows that p", I appear to say "Bob believes that p, he is justified in believing p, and I myself believe p". Where am I wrong ?
    – user47679
    Aug 3, 2020 at 20:51
  • @user47679 If you know that p, and you know that Bob justifiably believes not-p, would you say that Bob knows that not-p?
    – Eliran
    Aug 3, 2020 at 21:04
  • I would not say that he knows that not-p.
    – user47679
    Aug 3, 2020 at 21:09
  • @user47679 So you agree that justified belief is not sufficient for knowledge.
    – Eliran
    Aug 3, 2020 at 21:27
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    Thank you very much, the examples you gave were very useful. I marked your answer as accepted.
    – user47679
    Aug 5, 2020 at 16:03

The standard response has it that justified belief falls short of knowledge because you could be wrong anyway, and you cannot be said to know what is in fact false (even if the latter is behind the veil of ignorance) without stretch. (Your self-ascription is predicated upon JTB, by the way, to the extent that "I believe x and I am justified in believing x" precludes "but x is false".)

  • This is indeed the standard response I have found in the reference articles (namely in the SEP). But I struggle to understand it. Indeed, if justification is not enough to conclude about the truth of a proposition, then on which basis can we say that a proposition is true ?
    – user47679
    Aug 3, 2020 at 11:47
  • I think you'll see through it by distinguishing knowledge attribution from actual knowledge.
    – Turtur
    Aug 3, 2020 at 12:04

What you call "justification conditions" is not strong enough to guarantee truth. In other words, you can be very justified in believing something although it is false. This justification may be spent by a wonderful evidence you have.

Andrew Wiles, who gave a proof of Fermat's Last Theorem (FLT), has very good evidence that there are no a, b, c in N, such that a^n + b^n = c^n, n>2.

He and his few colleagues who fully understand this proof are excellent candidates for people who know FLT. However, albeit the wonderful evidence, FLT could be wrong. In this case, they wouldn't have known.

  • Thank you for your answer. Does it follow from it that truth requires certainty ? In case the answer is positive, then why does the technical definition of knowledge as JTB seem to be so distant from the common use of the word "knowledge" ?
    – user47679
    Aug 3, 2020 at 13:25
  • No. A belief is, as a matter of fact, true or false. However, you can be more or less certain about the truth or falsehood of an issue, according to whether you have better or worse evidence for it. Certainty/evidence comes in degrees and is a characteristic of your belief, not of truth.
    – Mr. White
    Aug 3, 2020 at 13:56
  • I see. So strictly speaking, I can only say that I know p if I am certain (to the highest degree) that p is true. Otherwise I would just be expressing justified belief, because it would be possible for p to be false even though I have some justification for it. Am I right ?
    – user47679
    Aug 3, 2020 at 14:29
  • According to the approach we discuss here, in order for you to know p, you must have an excellent evidence (justification) for the truth of p and -- independently of that -- p must be true.
    – Mr. White
    Aug 3, 2020 at 14:33

We want to distinguish between our use of "know" as when people say, "Oh, I just KNOW it's true," when a missing person is feared for (for example) from when they use the concept of knowledge as dependent on actual truth instead of desired (or perhaps ideological) truth, and use this to actually reach out publicly to either share knowledge or apply it. There is a generic strength of belief common to both uses of the word/concept, because some beliefs are strongly held on feeling, others due to epistemic reasons. These latter are at least, then, epistemic ideals. The case where someone says they know, when not only is their belief false but not even sufficiently justified, is like the empty case of the strength conditions.

For example, strong ideologies are grounded in the pure strength of deductive inference, in the wish to unite all explanations in one ultimate rule, often from some incredible premise (e.g. the Nazi idea of being hereditarily destined to make the greatest scientific and technological progress, a belief that they "knew" was true). But our questions are not asked like that; there is no magic moment when one answer is literally that ultimate. Our knowledge is beholden to truth enough for us to emphasize the aim for truth in statements of knowledge, even in the wishful cases (we do wish we knew, and that what we wish overall is true...).


Truth is what is real. If something affects everyone in some predictable way, then it's real. A statement that is consistent with reality is objectively true. Otherwise it is objectively false.

A person may (often does) hold many beliefs that are objectively true, and others that are objectively false.

Rationality means ability to explain your beliefs based on the first paragraph. Being able to explain a belief is what makes it justifiable.

Being fully rational, only holding the explainable beliefs, their explanation based on everyone sharing the same objective reality, the same universal truth, makes for self-actualization. For a sagehood, being enlightened human we all It makes for a sagehood, it makes for a human that everyone of us should have been -- kind, understanding, and treating everyone with compassion, dignity and respect.

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