When one knows something, do they also know it for certain? Or is knowing distinct from knowing for certain? I find a statement like, "I know X, but I don't know X for certain" to be pretty much a contradiction. But perhaps some philosopher has made a distinction between the two. If so, I would like to read such a text.

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    Basically anything by Karl Popper. We can have certainty about — basically nothing. Yet we have knowledge.
    – Dcleve
    Dec 23, 2023 at 1:03
  • "P but I'm not certain that P" is taken by some, but not all, to be a Moorean sentence (the original type is saying "P and I don't believe P"). The SEP has a dedicated entry on the topic of certainty. For more on Moorean sentences in this connection, see here. Dec 23, 2023 at 1:21
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    Real-life statements and vocabulary tend to accept a part of blurriness by default, so unless you're in a context where vocabulary has been defined precisely and explicitly (which is typically the case for domain-specific vocabulary and nothing else), then there is probably no contradiction because "I know X" cannot be taken for more than "I more or less have somewhat enough confidence in X to behave as if X is true"
    – Stef
    Dec 23, 2023 at 17:04
  • "I have opinions, strong opinions. But I don't always agree with them." - George Bush
    – Scott Rowe
    Dec 24, 2023 at 0:09
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    It's only a contradiction if you insist it's not simply sloppy speech. I see the Question as wholly dependant on the specific meaning of the underlying language, which to me puts on a par with asking 'how long is a piece of string?' or challenging the one-armed angler who holds out his limb and says he caught a fish 'that big.' Dec 24, 2023 at 18:52

8 Answers 8


A huge amount has been written about certainty, so you can readily find more-than-enough reading material on-line. In summary, yes there is a useful distinction between knowing and knowing for certain. We use the word 'know' to serve a wide variety of purposes, and sometimes, when we need to be clear about how we are using the word, we supplement it with other words, as in 'as far as I know'. Often we might say we know something when in fact we are mistaken. I might say 'I know the keys to the green Bentley are in here somewhere (here meaning the map room in the west wing) because I left them here after yesterday's meeting with that horrible reporter from the BBC' only to find that I had actually left them with the second under-chauffeuse. Or I might say 'I know my staff respect me, because they always call me "Sir"' when in fact my staff, to a person, consider me a pompous idiot. I am sure that with a few minutes' thought you would be able to imagine other circumstances in which you might use the word 'know' in a loose sense that does not imply certainty, and that is why the phrase 'know for certain' has useful work to do at times.

  • I'm certain that it is a useful distinction.
    – Scott Rowe
    Dec 24, 2023 at 0:34
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    Can't you also be mistaken when you say you know something for certain?
    – Barmar
    Dec 24, 2023 at 4:09
  • @Barmar I know for certain that I am often mistaken.
    – Scott Rowe
    Dec 25, 2023 at 15:10

If you 'know' something based on evidence, then Karl Popper is probably your man. However, Descartes 'Evil Demon' argument still applies: it is always possible that you are subject to a 'man in the middle attack' where all your experiences are modified so they do not reflect reality. We are unlikely to catch the demon doing their job, but it does set a limit on what 'knowing' can be.

Suppose we add non evidence-based beliefs, such as "I know that my Redeemer liveth". This is not the evidence-based "My Redeemer looked fine and healthy last time I saw them, and I am pretty sure nothing has happened to them". The belief is coming from within, or without, or wherever.

You might suppose a scientist must then ask "How do you know this?" and be dissatisfied with the answer. This seems not to be the case. I have known scientists who believe in God, and do good work. Aristotle recognised the importance of considering opposing ideas without favouring one over the other. If you can believe in transubstantiation, you may find it easier to accept wave-particle duality.

  • Your last section describes - or even votes - for internally splitting-off the opposite domain each time. - Your final sentence seems to be a rather abridged interpretation of quantum mechanics. In contrast, transubstantiation looks like pure magic.
    – Jo Wehler
    Dec 23, 2023 at 19:41
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    @JoWehler I am not sure I propose anything. I note others do not rely solely on evidence. I do not exclude them from this discussion but I am not one of them. Whereof I know diddly squat, thereof I should keep my gob shut. But the ability to consider something that does not fit common sense can have its uses. Dec 24, 2023 at 9:48
  1. Colloquial language often makes a difference between “I know” and “I know for certain”. The latter formulation puts more emphasis on my conviction to know. The answer of @MarcoOcram provides many examples from colloquial language.

    In a more academic context, e.g., in scientific writing, the expression “I know” is not gradable.

  2. But there is a long discussion about the conditions which the statement “I know” must meet. The traditional conditions are:

    • The statement must be true,

    • I must believe in its truth,

    • and I must must able to give arguments for my believe.

    See Plato Theaetetus. But even this combination of criteria has been questioned by the Gettier cases, see Gettier problems.

  • The truth of a statement can never be established with certainty. See Karl Popper on all of empirical knowledge, then consider logical pluralism vs. rational knowledge. If we cannot say any cases satisfy your first bullet then it is an impossible standard. When logical absolutism is not available for knowledge, then knowledge MUST be gradable.
    – Dcleve
    Dec 23, 2023 at 15:49
  • @Dcleve Popper did not say “The truth of a statement can never be established with certainty.” Of course we can know the truth or falsehood of a sentence like “Today there is snow in Manhattan” – a singular case. Likewise we can know the truth or falsehood of mathematical propositions – covering infinitely many cases. (1/2)
    – Jo Wehler
    Dec 23, 2023 at 18:29
  • @Dcleve Instead Popper deals with scientific theories: “Scientific theories, for him, are not inductively inferred from experience, nor is scientific experimentation carried out with a view to verifying or finally establishing the truth of theories; rather, all knowledge is provisional, conjectural, hypothetical—the universal theories of science can never be conclusively established.” see plato.stanford.edu/entries/popper/#BasiStatFalsConv. Conclusion: There is no knowledge in scientific theories, but “conjecture and refutation”. (2/2)
    – Jo Wehler
    Dec 23, 2023 at 18:31
  • We cannot know with certainty whether there is snow today in Manhattan. We do not have two classes of access to the world. We have NO direct knowledge. All our knowledge of the world comes thru indirect realism, where the meaning of our data inputs is only established thru conjectures and refutations. That some of this happens in our unconscious neurology does not change this limitation of indirect realism. And the unreality of some of our unconscious conjectures has been repeatedly demonstrated in lab experiments (and these experiments are the basis for delusionist views of consciousness)
    – Dcleve
    Dec 23, 2023 at 19:40
  • @Dcleve but on some days there is not a snowball's chance in Hell that there is snow in Manhattan, so we can be pretty much sure of it.
    – Scott Rowe
    Dec 24, 2023 at 0:37

Is there a difference between knowing, and knowing for certain?

Not really, no.

If you know that p, then p is true. Being certain that you know that p won't make any difference as to the truth of p and therefore as to whether you do or you don't know that p.

The only difference will be how you feel about it. You can be certain that you know that p and yet fail to know that p, including because p is in fact false, so that it would be impossible to know that p, for certain or not.

To talk of knowing for certain betrays a fundamental misunderstanding, on a par with the use of the expression "absolute truth", as if there were degrees of truth. The expression is shown to be just ridiculous when we make the parallel with a similar expression: I know but I'm not sure. Imagine anyone claiming that! Well, if it made sense to claim to know for certain, it would make sense to claim that you know but you're not sure that you really do.

We can also compare with the notion of belief. We can believe that p and be more or less certain that p is true. We can also truly believe that p even if p is false. We cannot similarly truly know that p if p is false. This should be enough to dismiss the various attempts made by academics to reduce knowledge to a belief.

Being certain is a psychological condition, which in itself doesn't prove that you know. It even suggests the opposite, for it just means that you believe that you know, which is as good as an avowal that you don't know because you merely believe that you know.

Obviously, simply stating that you know won't prove that you know either.

Humans are under pressure to claim knowledge. If you compete for a job, you better be inclined to claim that you know how to do it. This extends to academics. Imagine an academic logician admitting as an introduction to a paper on formal logic that he or she doesn't know how logic really works!

There is a certain ambiguity in our claim to knowledge. I can claim very plausibly that I know how to multiply numbers, a claim which only means that I know what I would do if I were to multiply some two numbers. Whether the result would be correct is anybody's guess. Further, what I think I will do at some future time is clearly a belief, and therefore not any sort of knowledge. Being right most of the time doesn't change the situation. Being certain about what we believe doesn't make any difference.

A specialist of Quantum Physics can claim very plausibly that he or she knows Quantum Physics. Whether Quantum Physics is a true description of the real world is anybody's guess. Further, what the specialist of Quantum Physics thinks he or she will do at some future time when required to apply his expertise is anyone's guess.

Talk of knowing for certain is likely the consequence of the intense social pressure to claim knowledge. Presumably, adding "for certain" to any claim suggests self-confidence and may invite trust on the part of the addresses. It probably has some rhetorical effect, but doesn't improve our prospect of knowing what we claim to know for certain.

People who believe that p and are certain about it, are also likely to say that they know that p. There is even, very plausibly, a positive correlation between people being certain that p and p actually being true. Experts are usually more certain than the non-experts and are also presumably more likely to make claims that subsequently prove correct. This may be the main cause of the extreme confusion in the semantic of the verbs "to know" and "to believe".


You present an interesting variant of what is sometimes called a Moore sentence. These were originally of the form, "P but I don't believe that P," as if one were to assert something while denying that one believes what one is asserting. There are those who claim that there is a similar infelicity to, "P but I'm not certain that P," but the intuition here is less widely shared than with respect to, "P but I don't know that P":

Some philosophers have argued that certainty is the norm of assertion: one can assert that p just in case one is certain that p (Stanley 2008; Peterson 2019; Beddor 2020b). The norm is often defended by adapting Moore’s paradox to certainty (Moore 1942). According to the original form of the paradox, there is something infelicitous about assertions like, “It’s raining, but I don’t believe it is.” Defenders of the certainty norm of assertion argue that assertions like, “Dogs bark, but I’m not certain that they do,” are just as odd (Stanley 2008, p. 47). The explanation of this infelicity is then provided by the fact that certainty is the norm of assertion. When the speaker asserts, “Dogs bark,” but then goes on to deny being certain of that claim, she has undermined her own assertion, contradicting in practice what she has just said.

Williamson (2000) and Turri (2016) reject the certainty norm of assertion. Although a certainty version of Moore’s paradox does sound infelicitous, it doesn’t sound quite so bad as a knowledge version. Moreover, the oddity of the certainty version can be explained by the fact that there is generally “a reluctance to allow the contextually set standards for knowledge and certainty to diverge” (Williamson 2000, p. 254). That is, the standards for what counts as knowledge and as certainty typically match one another. Nevertheless, in some contexts we can pull them apart. When that happens, Williamson suggests that we assess assertions in accordance with the knowledge norm. Turri (2016) reports empirical results confirming that this is in fact the way ordinary speakers assess assertions.

"I know P but I don't believe it," "I'm certain that P but I don't know it," and so on, are variations on the theme that will evoke further such intuitions and their dialectic. So we might put, "P but I don't know it," on one end of a spectrum of oddity, with, "I'm certain that P but I don't know it," on another, with the flipped, "I know that P but I'm not certain about it," somewhere in between.

Incidentally, there is also a "paradox of dogmatism" in epistemology:

Saul Kripke’s ruminations on the surprise test paradox led him to a paradox about dogmatism. He lectured on both paradoxes at Cambridge University to the Moral Sciences Club in 1972. (A descendent of this lecture now appears as Kripke 2011.) Gilbert Harman transmitted Kripke’s new paradox as follows:

If I know that h is true, I know that any evidence against h is evidence against something that is true; I know that such evidence is misleading. But I should disregard evidence that I know is misleading. So, once I know that h is true, I am in a position to disregard any future evidence that seems to tell against h. (1973, 148)

Dogmatists accept this reasoning. For them, knowledge closes inquiry. Any “evidence” that conflicts with what is known can be dismissed as misleading evidence. Forewarned is forearmed.

The article continues, "This conservativeness crosses the line from confidence to intransigence," and presents some of the dialectic of higher-order evidence to represent, diagnose, and treat the given paradox.


English is ambiguous. Which version of "know" are you after? Perhaps refer to the assorted versions in a dictionary? Reworded a bit from Merriam-Webster:

  1. To perceive directly
  2. To understand
  3. To recognize the nature of
  4. To recognize as being the same as something you knew previously
  5. To be acquainted or familiar with
  6. To have experienced
  7. To be aware of the truth or facts
  8. To be convinced of the truth or facts
  9. To have practical understanding

'To know' is a verb: someone or something has to do the action of 'knowing'. For humans, I understand this to include an electro-chemical state in a brain. That action is fallible. It is also unreliable; e.g. we have all seen other people (and done ourselves) jumping to conclusions; unaware of, misunderstanding, ignoring or forgetting evidence; have biases; etc.

Feeling certain is also a verb: typically, someone is having feelings (physical sensations in their body) -- often as the outcome of a more or less unconscious decision process -- which they associate with certainty. It is entirely possible to feel 100% certain and be up to 100% incorrect (note Dunning-Kreuger).

Coming to a logical conclusion of certainty is again a verb, with its own set of limitations.

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    Dec 24, 2023 at 16:47
  • @Technophile "an electro-chemical state in a brain. That action is fallible. It is also unreliable" It's been very reliable all my life that I know pain whenever I am in pain. You think not? Dec 25, 2023 at 10:52
  • @Speakpigeon which specific version of "know" did you intend? There's feeling pain, intellectual understanding, knowing facts about it, etc. Re reliability, do you "know" pain (whatever you specifically mean by that) when asleep or unconscious? When excited or distracted? Ever forgotten about a bruise or ache when fully engaged in a game, movie or other such activity? Dec 27, 2023 at 23:25
  • @Technophile "which specific version of "know" did you intend?" I'm sure "I know pain whenever I am in pain" is good English and 100% clear. So you don't know pain when you are in pain? And you don't know you are in pain when you are in pain? Dec 28, 2023 at 16:34
  • @Speakpigeon yes, it's in English. Due to the highly ambiguous nature of English, that is not at all the same as clear. Please see the list of possible meanings in my post. Feb 17 at 18:07

Commonsense is enough to answer this question. We all know that the term know / knowledge varies when the subject changes. And the word 'certain' is often used for emphasis. So, when these two words are combined, the meaning definitely changes.

Now, if you want to take a closer look at this question, you don't even need to consider other things for this. You just check yourself. 'I know myself' means that I know something about myself. It won’t indicate the reply after self realization. So, if this is the case when we consider our body, what will be the case of other bodies or objects? 'I know a thing' only means that I know only some properties / qualities of that thing ... and it often implies only a peripheral vision of that thing.

  • "We all know" is an appeal to vague authority. Who, specifically, is "we"? Through what means do "we" "know"? Dec 27, 2023 at 23:28
  • @Technophile: I mean almost all of us. Definitely it means exceptional cases are rare. Dec 28, 2023 at 4:53
  • sorry, I will clarify what I meant by "who.. is "we"". There are estimated to be over eight BILLION people on this planet (plus a few in orbit), ranging in age from being born to over 100 years in age. Probably a majority do not read, write or speak English. Some in comas. Given that context, please explain how you 'know' what "all of us" know or think. How exactly did or could you find out what everyone, including the inhabitants of China and India (the majority of humans), the Himalayas, tribes in the Amazon rainforest etc. know and think? Which "us" or "we", exactly? Jan 8 at 18:12
  • @Technophile: I meant humans in general. Not all of us who live even on this earth. Sometimes you may find some unusual cases in the Himalayas as well. Jan 9 at 1:47
  • I don't know what "humans in general" means. What percentage of the population are you claiming to speak for? How many have you actually checked with? Feb 17 at 18:12

The statement I know X, but I don't know X for certain is a contradiction. Hegel and Marxism have a lot to say about contradictions.

A kind of summary of the argument: To understand the nature of contradictions we first need to differentiate bewteen the two types of contradiction: Objective and Subjective Contradictions. Formal logic describes the subjective contradiction, a contradiction for us. It is subjective because we look at abstract things (in our minds) and not concrete objectively existing things. These abstract objects behave static in our mind. They do not change.

Dialectical Logic, on the other hand, describes objective contradictions. This are the contradictions that exist objectively in nature. Now, you might be thinking: that's not possible! This is because you are thinking in subjective logic, your mental abstractions of the things are static, they do not change. But concrete things do change all the time! Like the Ship of Theseus things are constantly becoming something else but at the same remaining as a continous whole. So a statement like X is up and down is true when we consider the fact that if we hold up something (x is up) and we let it go, it will fall bacuse of gravity, then becoming X is down. When we consider both states as part of a succesion objective contradictions are not incorrect.

To learn about the certainty of knowledge from a dialectical perspective, I reccomend Chairman Mao:

  • "The only thing I know is that I know nothing." - Socrates
    – Scott Rowe
    Dec 24, 2023 at 0:11

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