Everyone starts out at 0 and if anyone wants to say that they know something they have to prove that claim. Is the counter to this person claiming that they know something logically correct?

Person 1: 'I know that the sky is blue.'

Person 2: 'You cannot prove that we are not in a simulation and that anything is real, hence you cannot prove that your claim was true.'

Writing this I feel that it is a logical fallacy, but I am incapable of thinking it through myself. Could anyone tell me if this is correct or not?

Edit: Thanks for all the replies. You guys said alot of interesting things I'll have to think about


8 Answers 8


If a person claims to know anything could it be disproven by saying 'prove that we are not in a simulation'?

Yes, and this his is a very simple logical point.

"I know that p" implies "p is true". This is fundamental to the concept of knowledge.

And if p is true, then there is no possibility that p is false. Thus, if I know that p, then there is no possibility that p be false. However, if you don't know that there is no possibility that p be false, then you cannot exclude that p is false, and therefore it is possible, for all you know, that p be false, which falsifies your claim that you know that p is true (see note on possibility).

Please note the distinction between not knowing that there is no possibility that p is false, and p actually being false, just as there is a distinction between ignoring that p is false, and p actually being false.

Thus, an argument from ignorance can only at best falsify a claim to knowledge, not whether what is claimed to be true is in fact false.

Another crucial point is the question of what it is we are talking about when we claim to know that p is true.

The claims we make need to be interpreted in context. It may be true that I know that John Fitzgerald Kennedy is dead, but it wasn't true in 1962. That is, in 1962, the same claim would have been false, presumably.

That being said, there is one context that doesn't suffer interpretation: reality. While there are different times, different places, even possibly different John Fitzgerald Kennedy, there is one reality and there only one reality.

We can argue for example that even if we live in a simulation, it is possible to know that the sky is blue since we would in effect be talking of the simulated sky being blue, which may indeed be simulated as blue.

The same goes for, say, Quantum Physics and love. We are making claims about things as we think of them. Thus, my simulated mind may well represent correctly the simulated universe as obeying the laws of Quantum Physics as I think of them, provided the simulation is such that the simulated universe does obey QM laws as my simulated mind would think of them.

However, it may also be that although Alice sees the simulated sky as blue, the simulated sky is in fact simulated as not blue at all. Maybe, in our simulated universe, blue is only in Alice's simulated mind. In which case, she doesn't know that the simulated sky is blue since it is indeed not blue at all.

In other word, a simulation would void the notion that we could possibly know anything.

The fundamental reason is this: reality.

As we think of it, there is just one reality. A simulated reality isn't reality. To know that p is to know that p is true in reality. Claims to knowledge are thought of as true of reality, never of some simulated reality. This is the context of our interpretation of any claim to knowledge. And in a simulation, all we could possibly know would be the simulated reality, not reality.

And this would in effect void our notion of knowledge.

We all claim to know things all the time and very often when we don't in fact know what we are talking about. For example, some Joe may claim to know that monkeys don't fly. Suppose now that the next day a new species of monkeys is discovered and that these monkeys do fly! This would falsify the claim that monkeys don't fly and therefore Joe's claim that he knew that monkeys don't fly.

However, the most important point is that although Joe's claim is falsified only the next day, his claim was in fact already false the moment he made it. Thus, Joe didn't know, and never knew, what he had claimed to know that monkey didn't fly.

Usually, we genuinely believe we know what we claim to know. Yet, experience shows again and again that we can be wrong, and it is the simple possibility of being wrong, even if it is that we might be wrong, which falsifies our claims to knowledge.

I know that p implies that p is true. If I know p, then p is true. And if p is true, then there is no possibility that p is false. But if I don't know that there is no possibility that p is false, I don't know that p is true, which falsifies my claim to knowledge.

That in itself won't stop people claiming they know. This is entirely a practical matter.

In effect, whatever we believe we know to be true may in fact be false and there isn't much we can do about that. Most of the time, however, if you are somewhat conservative and prudent and reasonable in whatever claims you make, you won't be contradicted by the facts of the matter.

It is a fact of life that we will claim that p whenever we happen to believe with a high enough degree of confidence that p is true.

Thus, even though most of the time we don't know that p, we will nonetheless claim that p just because we believe that p, and because from experience we believe that most of the time, we won't be contradicted by the facts of the matter.

In practice, it is worth claiming you know all sorts of things again and again, even if it is not true that you know them, because if you are reasonable in your claims, experience shows that you won't be contradicted so often as to damage your standing in society. In other words, we don't feel like we need to fuss too much about our claims to knowledge.

This only apply to everyday life but it is everyday life that drives the way we use basic words like "know" and "believe". Hence, we will use the word "know" even when in fact we only believe, and this merely because we also believe we will get away with it, at least most of the time.

There is another aspect to the question, though, somewhat more subtle, but your question is right on spot.

An argument from ignorance should identify a possibility and not everything will do in that respect. For example, if you say, well, you don't know that p because you don't know that it is not possible that q and not q. This wouldn't work because in fact we do know that it is not possible that q and not q.

There is another case of ineffective argument from ignorance and this is when the situation that you are supposed to ignore whether it is possible or not just doesn't make sense. It is my opinion that the notion that we are in a simulation is nonsensical.

However, irrespective of whether my opinion on this is correct or not, the fact remains that nonsensical situations are ineffective for an argument from ignorance. We need to understand what it is we are supposed to not know that this isn't the case. The reason for that is simple. If we don't understand what is the situation involved in the argument, then we won't be able to decide whether we actually don't know that it is not the case.

Note on possibility

The notion of possibility I am referring to here is epistemic possibility: p is possible from your point of view whenever you don't know that p is false.

For details, see https://en.wikipedia.org/wiki/Epistemic_possibility.

Any notion of probability is entirely irrelevant to epistemic possibility. Epistemic possibility is a direct consequence of our notion of knowledge. If you don't know that something is false, then, for all you know, it may be true, i.e. it is possible.

  • 11
    why have you accepted this awful sprawling opinion.
    – user38026
    Commented Aug 16, 2019 at 15:52
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    There's a decent logical argument in here, but it's not connected to the original question.
    – industry7
    Commented Aug 16, 2019 at 20:33
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    "if you don't know that there is no possibility that p be false, then you cannot exclude that p is false" - No. This is the fundamental fallacy that underlies both Pascal's Wager and the false paradox "all hypotheses are false." Commented Aug 16, 2019 at 21:53

The issue here, as it often is, is that colloquial English is horribly ambiguous, which makes any sort of precise and rigorous discussion difficult. But with sufficient effort, it is possible to make claims precisely, and once you do that, the problem disappears.

Alice states that the sky is blue. Bob states that we live in a simulation. Let's assume for the sake of argument that Bob is correct. In that case, then the sky exists in a simulation...and the simulated sky is blue.

Consider these claims:

  • P1: The sky in our reality is blue
  • P2: Reality as we experience it is a simulation
  • P3: There exists a sky in the world outside the simulation, and that sky is blue

Alice is claiming P1. Bob states that P2 may be true, and therefore Alice can't know P3. Moreover, Alice says she knows P1, while Bob objects that she can't prove P3. While the exact definition of what constitutes knowledge is debated ("justified true belief" is a decent approximation, even if it has some issues (Gettier 1963)), the standards for proof are higher than the standards for knowledge.

Alice and Bob aren't having the same conversation; they just think they are because they didn't properly define their terms.

  • i'm in agreement with this answer, but you need some references in lieu of seeming too clever
    – user38026
    Commented Aug 14, 2019 at 20:49
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    Gettier himself provides a few references for the JTB definition, but I'm pretty sure it's (almost?) as old as the field of epistemology itself.
    – Kevin
    Commented Aug 14, 2019 at 22:55
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    I don't think that P3 accurately states OP's problem. OP's version of P3 seems to be "This reality's sky isn't really blue because it's only simulated to be blue." It's a little equivalent to saying "Blue doesn't really exist in the simulation". You might as well say that the sky, Alice, and Bob don't really exist in the simulation - only simulations of them do.
    – Beanluc
    Commented Aug 15, 2019 at 18:25
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    @Beanluc Refusing to accept that "blue" has any meaning in the context of a simulation seems utterly silly to me. It means what it is being used to refer to, regardless of whether that something is real. (The same goes for the word "sky".)
    – Brilliand
    Commented Aug 15, 2019 at 21:50
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    @Beanluc Considering that everything that Alice has perceived as blue in the past would have also been part of the simulation, it would be easier to argue that the only way "blue" has meaning outside the simulation is if the real world resembles the simulated one sufficiently closely. Alice's reality definitely has skies and blue. The "real world" may not. Either way, the important thing isn't that Bob's position is P3 specifically; the important thing is that it isn't P1. When Alice says "sky", she means "that thing, up there, whatever its exact nature may be." Bob means something else.
    – Ray
    Commented Aug 15, 2019 at 22:05

Consider what the second person said:

You cannot prove that we are not in a simulation and that anything is real, hence you cannot prove that your claim was true.

This may be an example of an argument from ignorance. Here is how Wikipedia describes it:

Argument from ignorance (from Latin: argumentum ad ignorantiam), also known as appeal to ignorance (in which ignorance represents "a lack of contrary evidence"), is a fallacy in informal logic. It asserts that a proposition is true because it has not yet been proven false or a proposition is false because it has not yet been proven true.

Wikipedia further explains what is wrong with such arguments:

This represents a type of false dichotomy in that it excludes the possibility that there may have been an insufficient investigation to prove that the proposition is either true or false. It also does not allow for the possibility that the answer is unknowable, only knowable in the future, or neither completely true nor completely false. In debates, appeals to ignorance are sometimes used in an attempt to shift the burden of proof.

Although Wikipedia considers this an informal fallacy Douglas Walton provides an example where an argument from ignorance is appropriate:

There seem to be many cases where, as a presumptive guide to action, the ad ignorantiam argument appears to be quite reasonable. One cited in (Walton, 1996, p. 86) involves the use of the common rule of safety in the handling of firearms: if you do not know for sure that a weapon is unloaded,you should act in accord with the presumption that it is (or may be) loaded.In a given case, if I walk up to the firing range and pick up a pistol, the right thing to do before waving it around is to open the chamber and check to be sure that the weapon is unloaded.

Walton is referencing his book length treatment of this argumentation strategy: Walton, Douglas: 1996, Arguments from Ignorance, Penn State Press, University Park, Pennsylvania

Walton, D. The Appeal to Ignorance, or Argumentum ad Ignorantiam. Retrieved on August 14, 2018, from Douglas Walton's site at https://www.dougwalton.ca/papers%20in%20pdf/99ignorantiam.pdf

Wikipedia contributors. (2019, July 23). Argument from ignorance. In Wikipedia, The Free Encyclopedia. Retrieved 14:49, August 14, 2019, from https://en.wikipedia.org/w/index.php?title=Argument_from_ignorance&oldid=907463760

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    One cannot say x is a logical fallacy & then say there are cases were the fallacy is okay. In the Douglas Walton example there is a lack of understanding concepts. Some people apparently belief any wording involving the words if . . . Then is a conditional argument. This may be true SOME OF THE TIME. To say that any occasion you see IF . . . THEN structure is a logical statement or an argument is objectively false. In the Watson example the If . . . then structure is INFORMATONAL or INSTRUCTIVE. The intent of the message is NOT to make a logical true or false proposition.
    – Logikal
    Commented Aug 14, 2019 at 15:28
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    In other words follow these procedures while on the firing range or else you have no business here. There is no argument present in the example dispite the Mathematical logic habit of people thinking there is a legitimate proposition present because they LITERALLY SEE the words IF followed by THEN. This show Walton lacks understanding of what propositions are and how argument work. He blundered as there is no case of the fallacy there because he ignores content and intent. What is BEING EXPRESSED is follow the directions or else. . .
    – Logikal
    Commented Aug 14, 2019 at 15:31
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    I cannot help but thinking that "there may have been an insufficient investigation to prove that the proposition is either true or false. It also does not allow for the possibility that the answer is unknowable" means you cannot prove it, period. That's not a defect of the argument; it is the argument. Commented Aug 15, 2019 at 9:56
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    Well, it has been shown that "the sky is blue" cannot be proven, which was the skeptic's claim. I cannot see a fallacy. (In particular, the skeptic's claim was not "the sky is not blue".) Commented Aug 15, 2019 at 14:42
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    You quote wikipedia as saying that the Argument From Ignorance is when someone says that something is true because it hasn't been proven wrong. But here, the claim is that something hasn't been proven true, therefore it hasn't been proven true. That's not the fallacy of argument from ignorance. Commented Aug 16, 2019 at 0:53

No, that argument is not logically correct because knowledge need not be absolute, but can be relative. To know the sky is blue means to connect the perception of the atmosphere that we call the sky with the quality we call blue. If the world is a simulation or not, the two perceptions are relatively the same.

This is common in science: The work done raising an object from point A to point B depends only on the difference in the distance from A to B and not where the object was manufactured. Finding the doubling time of a culture of bacteria depends on the amount of bacteria at time A and time B and not on how many bacteria were initially present etc. Likewise, the appearance of the sky depends on our average perception of it and not on the history of our universe.

  • "To know the sky is blue means to connect the perception of the atmosphere that we call the sky with the quality we call blue." So, now our subjectivity is the reference? You know that the sky is really blue?! What about people who see the sky not as blue but as green? Is the sky therefore both really blue and really not blue?! Commented Aug 18, 2019 at 9:52
  • @Speakpigeon Of couse subjectivity is the reference it always has been. One may think a concert is loud while a deaf person thinks it's quiet. One may see a red and green apple while a colour blind person sees two identical apples. One may find ambient temperature cool, while someone with thyroid problems would find it too hot. We don't call the latter liars or wrong, it's simply their reality as a result of their perception. Furthermore we may be convinced a flower has yellow petals, but insects may tell us they are possibly blue and striped because their perceptions allows for that truth.
    – Cell
    Commented Aug 18, 2019 at 11:17
  • You really think this is what people mean by the word "knowledge"? So, the Sun turns around the Earth?! Electrons don't exist since I can't see them? I know you don't exist?! Reality is only that which I have a subjective experience of?! You think this is what people mean by the word "reality"?! Commented Aug 18, 2019 at 11:44
  • @Speakpigeon As a researcher in science, yes that's what people mean by the word "knowledge" i.e. we only know that microorganism Y produces protein X only when we perceive it's presence by some technique that we can see via our senses. All you are doing is listing knowledge aquired through inference of indirect perception, but that isn't relevant in this question nor does it change my argument. FYI unless you have read of or seen the experiments that imply the existence of electrons you don't "know" they exist you just believe the scientific authority. Just to nitpick a little.
    – Cell
    Commented Aug 18, 2019 at 11:54
  • You're not addressing my remarks. Commented Aug 18, 2019 at 18:35

This is basically equivalent to solipsism, the idea that the only thing you can be sure really exists is yourself, and everything else could be just a figment of your imagination.

It's fundamentally irrefutable: any argument can be dismissed as "That's not real and I could imagine something different tomorrow", "But that's just what the simulator told you to say" and so on.

At the end of the day, this is an intellectual dead-end. If you insist that you can't know anything because you might just be imagining it or it might just be a simulation, then you have two options: either give up and don't think about anything, or accept that your imagination/the simulation are the only things you can reasonably equate with existence and reality and to think and reason about them on that basis.


A person who is claiming to know something is asserting that they meet the criteria for claiming knowledge. There are three requirements for a person to "know" something:

  1. They must believe the thing they claim to know.
  2. They must have an objectively reasonable justification for that that claim.
  3. The thing they claim to know must be true.

So if a person asks you, "Does anyone know why John wasn't at work today?" they're not asking if anyone has some kind of proof. They're not asking about metaphysical certainty. They're simply asking if anyone has a belief about why John wasn't at work today that's based on sufficient reason and evidence to justify the person asking the question also accepting that belief.

In your example, the person claims they know the sky is blue. You claim they are wrong. Which of the three requirements are you claiming to know is false?

It can't be 1. You haven't made any argument that they don't believe the sky is blue. And it can't be 3. You haven't made any argument that the sky is not blue. That leaves 2. You need to show that they have no objectively reasonable justification for that claim.

But since your argument is not specific to their claim, you're essentially arguing that nobody ever has any objectively reasonable justification to accept any claim about the nature of the world. If that were true, we could not function in the world, that is, our reason would be useless for the sole purpose for which we employ it.

Update: It's been suggested that the OP really is suggesting that no person is ever justified in accepting any fact about the world as a predicate for further reasoning. That's just an obviously useless standard for accepting predicates for further reasoning.

Again, though, I don't think that's what the OP is saying at all. I think the OP is simply misunderstanding the requirements for declaring something knowledge. For example, thinking that proof or certainty are required. It is not. As I said above, it just requires a justified belief. That is, there must be some reason you accept it that is sufficient to justify that acceptance. You observe it. It has predictive validity. Those are objective reasons.

Believing something means you've accepted it to the point that you are willing to let it inform your actions. That you observed it and it has predictive validity makes it objectively reasonable to accept it to the point that you are willing to let it inform your actions. That is knowledge.

  • -1. you're essentially arguing that nobody ever has any objectively reasonable justification to accept any claim about the nature of the world. Yes. That's exactly the point that OP is making. You haven't addressed it at all, you've merely re-stated it and said "this would make me sad, so it must be false".
    – Brondahl
    Commented Aug 15, 2019 at 10:53
  • @Brondahl I'll address that point if you think it's applicable. But I really don't think the OP is making such a trivial, obviously rebuttable argument. See my update for why I think that misunderstands the OP. Commented Aug 15, 2019 at 10:55
  • I appriciate the detailed response. Though i did attempt to make the claim that humans can't know that we have knowledge. And your comment did give me useful insight. You did call it a useless standard and it might seem like that, but I focus on if we actually can have knowledge to begin with, and don't care if that is useless or not - it's irrelevant. I live my life by the axiom that 'my brain is and my feelings work'. I can't really prove that I know anything which is why I have to build it all off of an axiom, even though everything could be 'fake'. Commented Aug 15, 2019 at 11:12
  • @RolfHaugaland So I was right, you are simply misunderstanding what it means to "know" something. As I said, it's just a belief that has some objectively defensible basis. Believing something means you've accepted it to the point that you are willing to let it inform your actions. That you observed it and it has predictive validity makes it objectively reasonable to accept it to the point that you are willing to let it inform your actions. That's knowledge. See my "Does anyone know why John wasn't at work today?" example. Commented Aug 15, 2019 at 11:15
  • Then this confirms my point, they dont really know. Ofcourse as you said it is objectively reasonable to let that belief inform your actions, however at a foundational level you can't be certain and know that what you know is objectively true, right? Then again you couldn't live like this which is why I have the axiom. This could probably be considered to be unreasonably skeptic, but true nonetheless right? Also thanks for the well written replies Commented Aug 15, 2019 at 11:29

I don't know its name, but this is the same fallacy that underlies Pascal's wager and the false paradox that says all hypotheses are false. The false paradox says that for any hypothesis, there are an infinite number of alternative explanations, each with a small but finite probability, and the sum of those probabilities infinitely exceeds the probability of the original hypothesis. Similarly, Pascal's wager argues that the infinite value of eternal life times a small non-zero probability of God existing equals infinite value.

The fundamental fallacy here is that mere plausibility or the inability to disprove something implies a non-zero probability. This leads inevitably to the aforementioned paradox. And as we know, there are no paradoxes -- just flaws in our paradigms. If we reason instead that assuming a non-zero probability requires evidence, not mere plausibility, then the paradox goes away. Although we cannot prove that we aren't in a simulation, in the absence of evidence, we can treat the probability that we are in a simulation as zero and dismiss the suggestion out of hand. The same goes for God (sorry, Pascal) and Russell's teapot.

  • In a Bayesian framework, if you ever assign something a probability of zero, then no evidence can make you change that assessment (the posterior probability is the prior probability multiplied by an update factor, so if the prior probability is zero it doesn't matter what the update factor is). So assigning something a probability of zero means that you are sure that no matter your subsequent observations, you will still not believe it. This seems inconsistent with your model of probability. Commented Aug 17, 2019 at 8:27
  • @BenMillwood: I beg to differ. The probability of observing any specific hyperbolic trajectory is 0; but we observe hyperbolic trajectories. Therefore, we observe P=0 events.
    – Joshua
    Commented Aug 17, 2019 at 22:04
  • I think I disagree that we observe P=0 events because we actually don't observe precisely which trajectory occurred, we observe properties of it which hold for many possible "true" underlying trajectories, which together have P>0. Even if you don't buy that, it's still true that no Bayesian evidence can take a trajectory for which your prior was P=0 and update it to a posterior where P>0. Commented Aug 19, 2019 at 15:13
  • @BenMillwood In a Bayesian framework, if you receive new information that is totally outside the parameters of the analysis, you restart the analysis. Commented Aug 21, 2019 at 16:51

So, we've been having fun with this one. But your simulation friend is playing with dangerous toys.

Perhaps you saw the headlines along the lines of Physicists find we're not living in a simulation, and the huge amount of debate thereof about how they can't possibly prove that. Well perhaps not, but they can prove something. The universe can't be simulated by a Turing-equivalent machine or even the simpler forms of hypercomputers. The experiment showed the angular precision of the universe is infinite, and therefore any simulation must be doing arbitrary precision computations on reals.

Anyone who is not willing to take the plunge headlong into second order logic must concede that the universe is not a simulation. But if he does, he's picked a model that can't at once be sound, complete, and effective. If not sound, then no claim is true. If not complete, than we can't take a statement and say whether or not it even has a meaning. If not effective, than we can't verify proofs. That is, the very process of reasoning is broken.

My knowledge has failed me; and the end of this answer was unreliable, so I have deleted it.

Unfortunately I've encountered an upending; relativistic physics is not actually in first order logic; and I don't know what this means for this question either. I would normally hold we have disproven simulation altogether; but somebody willing to postulate a large scale second order logic and hypercomputer will find himself on shakey ground because we don't know if the reasoning model he is using works. Most second order models are ultimately unsound; leading mathematicians to abandon them in the early 20th century and try to make everything work in first order.

  • Very impressive! But, the fact that we think we "reason" couldn't prove logically that we don't live in a simulation. We would need first to prove that we actually reason logically. However, if we lived in a simulation, our logic itself would be a simulation. Proving we're not living in a simulation using the simulation of logic we have because we live in a simulation wouldn't prove we don't live in a simulation. I would agree the idea that we live in a simulation is nonsense but for a different reason. Commented Aug 18, 2019 at 9:36

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