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Can we doubt all knowledge from all sources (perception, reports, and reason)?

Regarding doubting reason, reason can't be proven, it is preceived and judged instantly by our logic, but what if our logic is not true?

Did anyone ever doubt that much? What definitely proves or disporves his/her doubts?


If we can doubt them, why do we all depend on them? behave according to them?

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Philip Klöcking
    Jul 28 at 17:52
  • we all live in our reality bubble. some time a lot of people share same bubble, which we call our shared knowledge. reality varies according to people, but that doesn't mean someone is wrong. any statement is validated by society which may not be correct. remember copernicus's case. Jul 30 at 14:58
  • 1
    smbc-comics.com/comic/prove
    – Barmar
    Jul 31 at 20:02
  • It depends on how define knowledge
    – Ha'Penny
    Aug 5 at 9:43
  • I don't believe in nihilism.
    – BillOnne
    Aug 7 at 4:54

16 Answers 16

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To answer the question in the title:
Yes. That's a key trait of any good scientist.

To answer your last question in the body:
Because we have no better option to depend on or behave according to.

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    "That's a key trait of any good scientist", can we doubt that too?
    – AZeed
    Jul 27 at 20:00
  • @AZeed Yes, that statement is made from following the (doubtable) logic that we depend on and behave according to our imperfect knowledge because we don't have a better option.
    – TCooper
    Jul 27 at 20:12
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    Other than "Did anyone every doubt that much?" and it's immediate follow up which are unanswerable, the other questions within the body are simply extrapolations of the question in the title. I can't provide a better answer than what I have without falling into a cycle of circular arguments that result in the same answer as soon as the condition is realized and the circle is broken.
    – TCooper
    Jul 27 at 21:48
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    What a short but great answer!
    – stevec
    Jul 29 at 18:01
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    What philosopher started this? Every philosopher asks this question. It is a fundamental axiom of philosophy to start with this question. It's like asking who invented the "wheel." I don't think this answer needs to address that to be an adequate answer.
    – OCDev
    Jul 31 at 12:44
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You can of course do anything you want... But to doubt all knowledge is to indulge in radical skepticism, is it not? If we were all radical skeptics, then we'd be living in a world in which knowledge had no intrinsic value and could not be acted upon, and neither physics nor engineering would exist.

As convinced as I am of the value of hot & cold running water and lights at the flip of a switch, I would not want to live in a world where all knowledge is doubted.

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    "If we were all radical skeptics, then we'd be living in a world in which knowledge had no intrinsic value and could not be acted upon, and neither physics nor engineering would exist." This is just not true. Skepticism does not imply nor require non-action or non-investigation. I can (and do) acknowledge that the existence of this computer is provisional without refusing to type on it.
    – philosodad
    Jul 28 at 15:50
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    It certainly does in an industrial context. skepticism and underdetermination are two of the most powerful management tools to justify inaction on engineering problems. Take it from one who knows. Jul 28 at 17:37
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    Why would I believe a claim that a universal principle exists based on the anecdotal opinion of a person I've never met and can't evaluate over my own experience and counterexamples? Why do you assume that I am not also someone who works on engineering problems?
    – philosodad
    Jul 28 at 18:11
  • @philosodad, I make no such assumption. When dealing with managers responsible for minimizing lost time on a production line where one minute of such loss costs $1400, I never present anecdotal opinions, I present facts and data. Jul 29 at 4:37
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    wait all you like. Jul 29 at 18:45
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This question resembles the "Can we reach certainty?" question, answering with "No" is self-refuting because that would render the answer itself as uncertain, since it is knowledge too.

Similarly, if the answer to "Can we doubt all knowledge?" is "Yes", then the answer is itself uncertain, so it can only be "No", but only if you take first-order logic for granted, and there is no escape from doing so, first-order logic (non-contradiction, causality, logical operators, addition & subtraction...) are the foundation of all knowledge, the description of how existence works even (and not just our universe), you can't even formulate an objection to logic without using it. How can someone ask for evidence for logic while he can only judge the evidence as valid or not with reason (which is founded on logic)? the word "evidence" has no meaning outside logic.

Logic is the anchor of all knowledge, doubting it is an uncalculated intellectual suicide, and is a self-defeating idea.

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    "This question resembles the 'Can we reach certainty?' question, answering with 'No' is self-refuting because that would render the answer itself as uncertain" - there's nothing self-refuting there. And if you're uncertain about whether we can reach certainty, it doesn't make sense to then just conclude with certainty that we can reach certainty. That's not how logic works. "if the answer to 'Can we doubt all knowledge?' is 'Yes', then the answer is itself uncertain, so it can only be 'No'" - that's also not how logic works. And the better question is "should we try to doubt everything".
    – NotThatGuy
    Jul 28 at 13:17
  • @NotThatGuy In the contrary, that's how logic works, if X is false then not-X is true, if the answer to "Can we reach certainty?" cannot be "No" then it's "Yes", it cannot be "No" simply because you're either certain that we can't reach certainty, and that's a self-refutation, or you're uncertain therefore your answer is without value, so by excluded middle the answer is that certainty is possible, and this itself is an example of certain knowledge.
    – Amin
    Jul 28 at 16:05
  • @NotThatGuy In fact even if you answer with "I don't know", you acquire certain knowledge about the fact that you don't know.
    – Amin
    Jul 28 at 16:22
  • "it cannot be 'No' simply because ... or you're uncertain therefore your answer is without value" - just because you personally don't find value in an answer doesn't make it false nor is it a good reason to conclude that it's false. Truth doesn't depend on what you find valuable, and suggesting otherwise indicates a very flawed epistemology.
    – NotThatGuy
    Jul 28 at 17:05
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    @NotThatGuy saying "I don't know" is objectively not an answer of value, it doesn't create knowledge about whether certainty is possible or not, and that has nothing to do with my subjective judgement, moreover I already explained why "I don't know" is also self-refuting, because you acquire certain knowledge about the fact that you don't know. you have been claiming that my reasoning is flawed and that logic doesn't work like how I used it, but so far you didn't precisely indicate where's the flaw, you only tried to subjectivity my claim, please reply to argument with counter-argument.
    – Amin
    Jul 28 at 18:57
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If you doubt everything, wouldn't you doubt that you doubted everything? Wouldn't it be possible that you didn't doubt anything, then? But so if doubting everything logically transforms into believing everything, then hasn't a doxastic explosion occurred? And don't explosions go against the sense of difference between the presence or absence of their context's defining concept, in this case the concept of belief?

Or what is it to doubt something? Is it not to ask, "But is that true? How do I know that?" Are these questions meaningless? Did John Cook Wilson anticipate those who say that the concept of knowledge is not first grounded in other concepts, when he reasoned that:

Given that “our experience of knowing [is] the presupposition of any inquiry we can undertake”, Cook Wilson reasoned that “we cannot make knowing itself a subject of inquiry in the sense of asking what knowing is” [...]

Or what is certainty? And is the concept of clarity/precision of expression vague? If all these epistemic qualifiers are peculiar and amorphous enough as such, why act on dogmatic skepticism? Again, is the distinction between skepticism and infinite credulity, nonexistent in the limit? As if to say, "Everything is clear and certain," though at the same time, "Nothing is clear or certain"? Who in such a mental situation is to gainsay the dogmatist about more substantial questions?

The above was written as a series of questions in accordance with the concept of a skepticistic "game": the skeptic is a player who tries to only play questions on each of their turns, and the claimant has to play a question that "compels" their opponent to make an assertion. I typically assume that the skeptic proceeds by playing the specific question, "How do you know that?" so that the solution is for the claimant to ask, "What do you mean by, 'How do you know...'?" Having at least been "made" to assert a meaning-claim, the skeptic is thus drawn into the mystery of language, and not in the adolescent manner that they had approached this mystery beforehand.

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  • "If you doubt everything, wouldn't you doubt that you doubted everything?" - yes, so ask if there are any other things you're yet to doubt. "Wouldn't it be possible that you didn't doubt anything, then?" - how do you go from "doubt as many things as possible" to "doubt nothing"? If you doubt at least 1 thing, then it's trivially false that you "didn't doubt anything". "Are these questions meaningless?" - no. "what is certainty?" - what the dictionary says is a good start. "is the distinction between skepticism and infinite credulity, nonexistent in the limit" - no.
    – NotThatGuy
    Jul 28 at 12:30
  • @NotThatGuy, two things: if we doubt everything, then we doubt the theory of valid inference, so we doubt, "Doubting everything doesn't mean doubting nothing." Maybe it DOES mean that, here. Secondly: if doubt is an iterable propositional operator, then, "I doubt that I doubt that X," is "close to" (not identical with), "I believe that X." There is a paradoxastic logic to it that mediates the amorphism discussed above. Jul 28 at 12:38
  • "we doubt the theory of valid inference" ... but doubting does not equal rejecting, and you'd only have a problem if you treat doubt and rejection as one and the same. "'I doubt that I doubt that X', is 'close to' (not identical with), 'I believe that X'" - no. [If you're just going to play on technicalities to try to sound smart, as opposed to sincerely trying to understand the opposing position, then I don't feel the need for or benefit of bothering with a proper explanation of why you're wrong.]
    – NotThatGuy
    Jul 28 at 12:53
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    "Why do you ask 'What do you mean by "How do you know?"?'?"
    – philosodad
    Jul 28 at 13:04
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    @KristianBerry you're missing my point... my point is, once I can embed your question into another question, the skeptic is literally never forced to make any kind of a meaning claim. If you ask what I mean, I can ask what you mean by mean, etc.
    – philosodad
    Jul 29 at 2:51
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So I think I can be almost 100% certain of the following:

  1. The experience of being me exists.
  2. There is an explanation for the experience of being me that is not the experience of being me.

I don't know who the very first philosopher to doubt everything was, but skepticism has a long history that extends at least back to Pyrrho of Elis.

I can't speak for skeptics as a group, but for myself, I don't accept as absolute that I live in a physical reality populated with other minds. I behave as if this is true because as far as I'm able to determine, I can't behave any other way. The existence of physical reality is also the most parsimonious explanation for my experience.

I'm open to alternatives to the real world hypothesis, but so far I haven't heard one that I find convincing. Everything else follows as consequences of that hypothesis.

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    "I think, therefore I am" (and that's all one can really be sure of) was famously said by Descartes, although he may not have been the first to express that idea. The Wikipedia page on the topic lists some predecessors, although they seem to focus more on being sure you exist than that being all you can be sure of.
    – NotThatGuy
    Jul 28 at 14:32
  • @NotThatGuy I would consider the Pyrrhonists and the Academic Skeptics to have precedence over Descartes, but others might disagree. Also, Descarte is way too sure of his own existence.
    – philosodad
    Jul 28 at 15:19
  • @NotThatGuy: Is not not "I doubt, therefore I think, therefore I am." And OP doubts.
    – Joshua
    Jul 28 at 21:01
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Should we question all knowledge from all sources?

Yes. That's the only way one can rationally have confidence in one's beliefs.

Regarding doubting reason, reason can't be proven, it is perceived and judged instantly by our logic, but what if our logic is not true?

Not sure I quite got your question.

But different people will reach different conclusions given similar or the same information. So it stands to reason that the reasoning ability of one of those people is flawed (and you necessarily are one of those people for a wide range of beliefs).

If you don't question your own reasoning ability, it's much less likely, or impossible, that you'd find flaws in your own reasoning and you can't rationally be confident that your own reasoning ability is flawless.

We've come up with some principles of reasoning and we see how well these work or don't work when applied to different ideas. This can serve as evidence for or against you having good reasoning ability.

And yes, it can be quite hard to accurately judge whether your reasoning ability is flawed or not, given that you're judging this with your reasoning ability, and many people have quite a strong aversion to being wrong and have strong emotional attachments to some of their beliefs (meaning they'd be less open to seeing a flaw in their reasoning if their belief relies on that flaw).

Did anyone ever question that much?

It's not that much.

You should proportion your questioning to:

  • The importance of the belief.
  • Whether there are other plausible competing ideas. This can be judged to some degree by seeing whether a decent amount of people hold competing beliefs, but it might also be that the truth hasn't been discovered or popularised yet. And I don't mean you should go with what the majority thinks, but rather just that if a decent proportion of the population believes something, that might merit investigating it.
  • How well the evidence supports the belief compared to competing beliefs. But this is something you'd likely only know (and not deceive yourself about) after you've done some sincere questioning.

As an example, the existence of an afterlife and whether we dedicate our lives to some deity would be quite important, and there are certainly competing ideas and people on various sides claiming they have reasonable justification for belief. So regardless of which side you're on, you should probably question that to some degree.

The composition of a star on the other side of the universe, on the other hand, is probably not that important to anyone except astronomers, and there aren't many plausible competing ideas, so you can probably just go with what the scientific community says until you have a reason to question that belief.

What definitely proves or disproves one's doubts?

Nothing definitely proves or disproves anything.

But given sufficient evidence, we can be 99%+ confident that some belief is true.

There's always some uncertainty, but this mostly just means you should be open to the possibility that you're wrong.

If we can doubt beliefs, why do we all depend on them and behave according to them?

Because we've evaluated the evidence and concluded that what we believe is the most likely of all the competing ideas.

If you're not much more certain of your belief above others, then you should probably investigate further (to become more certain, of what you believe, or a competing idea) or account for plausible alternatives in your actions. If you're fairly sure that you turned off the oven, but there's a good chance that you didn't, it's probably best to check, because if you did turn it off, you (generally) only lose a bit of time, but if you didn't turn it off, checking is much more beneficial.


* I prefer "question" above "doubt", because questioning is an action, whereas doubting is how you feel about things you're uncertain about (at least by my definitions). You don't typically feel uncertain about a belief until you start questioning it, and you don't need to feel all that uncertain about a belief (but still be open to the possibility that you're wrong) after you've questioned it to a point where the belief has been sufficiently validated.

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  • A simple counter example always with CERTAINTY disproves a claim made. If I claim all swans are white and discover black swans in Australia the original claim about swans being white is with certainty false. We may have issues PROVING so claim true. Sometimes we have an easier time proving something false. When we show a counter example to a universal claim we show with certainty the universal is false. No way to doubt that. Thus certain knowledge must exist in our realm. We cannot say there are no absolute truths or certainty. Another term for these is objective truth.
    – Logikal
    Jul 28 at 12:12
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    @Logikal I never said "there are no absolute truths or certainty" in my answer (and absolute certainty supports my answer more than it rebuts it, because you can be absolutely certain while being objectively wrong). But if you just assert "obviously this is an absolute/objective truth so it needn't be questioned"... that's how you start and keep believing objectively false things.
    – NotThatGuy
    Jul 28 at 12:43
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    You ate using objective in the wrong context. You cannot be objectively wrong and make absolutely true claims. You directly state: "Nothing definitely proves or disproves anything. . "
    – Logikal
    Jul 28 at 12:49
  • You are using objective in the wrong context. You cannot be objectively wrong and make absolutely true claims. You directly state: "Nothing definitely proves or disproves anything." I am guessing your background is in science? A claim can be objectively false or objectively true. A claim cannot be technically wrong because again if a claim is sometimes true and at other times false then that claim is not objective. The claim is called contingent. So claims that have temporary truths are not objective even though they are unbiased or influenced by other humans. Contingent expresses temporary
    – Logikal
    Jul 28 at 12:56
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    @Logikal I see nothing in your comment that rebuts anything I've said.
    – NotThatGuy
    Jul 28 at 12:58
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Can we doubt all knowledge?

First, we could not logically claim that we know we don't know anything.

This leaves the possibility nonetheless that we may only believe what we think we know. Anything we think about the world outside our own mind is based on some impression in our mind, for example, visual percepts etc.

What remains is our knowledge of our own mind. For example, when I am in pain, I know I am in pain. I also know pain, or at least the particular kind of pain I am experiencing right now. This applies to any impression we may have.

Impressions usually suggest some conclusion about the world outside our mind. If I have the impression that I am looking at a tree, I may believe that there is a tree in the world outside my mind. This may well be false, as discussed by Descartes. So impressions are no reliable sources of knowledge about the world, even though absent any other information, we may decide that it is after all best to accept that the tree is real.

So impressions may be false and they certainly seem to be false on at least some occasions. Further, impressions do not tell us what the thing really is. Rather, they tell us what the thing looks like to us, so to speak. The point is that if our impressions are consistent in how they represent things in the world outside our mind, this should be good enough for using them to make decisions about what to do next, and hopefully survive to see another day.

So impressions may be false but it would be absurd to claim that we don't know what our impressions look like to us. By definition, an impression is what it looks like to us and so we cannot logically deny that we know our own impressions, even as we admit that they may not be true of the world outside our mind.

So, when we have the impression that we are looking at a tree, we do know that we have the impression that we are looking at a tree.

Another way to look at this is that our notion of knowledge is grounded in our epistemic relation to our own impressions. It is our own impressions that provide for us, or perhaps more accurately for our brain, the base line for our idea of knowledge.

Given this, we can interpret our tendency to sometimes claim knowledge in social contexts as a very natural survival strategy. We will have better prospects of surviving in a social context if we can convince other humans that we know stuff, though convincing ourselves that we know rather than just believe may turn out to be counterproductive and sometimes dangerous.

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No because that leaves us no knowledge to justify our rationales for doubting.

The idea we can or should assume a view from nowhere in the interest of neutrality is fundamentally nonsense.

If we must doubt everything we know, then we have no justification for positing meaning in our doubts either.

To reject all possible knowledge and understanding is not to think critically but to embrace all of ignorance.

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Actually it is logic that leads to doubt , so to doubt logic , one first have to doubt doubt. So it is vicious circle with no definite answer, One has to hold on to Logic, Faith , Love or Self belief to live life with some order or there will be chaos and nothing else.

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Can anyone who claims that everything is doubtable prove it rather than keep building endless layers of doubt? endlessly building layers of doubt means that we will never reach the proof that rational information is doubtable, rather it is obvious that rationally necessary claims have never and will never be disproven.

If someone doubts rationality itself, then he doubts it rationally, which means, again, he will doubt endlessly and will never prove his doubts, so there is no reason to doubt, otherwise, you become like animals, only basic thinking to achieve survival, mating and other needs.

If someone doubts perception, he should live within his illusions until he proves they are illusions.

If someone has another way of knowledge that contradicts some rational facts, he must prove it so that we start following him, otherwise, we will continue following our doubtable logic.


It is possible that science changes; because observation of repetitions is the base of all scientific facts.

But Mathematical truths are purely rational, so they will never change; and thus, aren't doubtable. The only Mathematical truths that may change are the complex ones, not because rationality is doubtable but because they are complex, which increases the chance of making mistakes.

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    Indeed your amateur intuition is on the right track, but it's much better to take home some well-known professional epistemology theory out of this which is Norzick's truth-tracking theory of knowledge in stead of the thousand years old JTB knowledge as discussed by one of my old post. As a byproduct, this theory reveals our ironclad modus ponens logic rule breaks down when dealing with epistemology/knowledge as Norzick had to reject epistemic closure therein... Jul 29 at 4:54
  • your first statement seems to rely on the law of excluded middle, which can't be taken for granted when you're trying to prove that endless doubt is not viable (because in doing so you rely on logic that you doubt in the first place). I am not familiar with epistemology enough to say, but from my standpoint, this seems similar to the Halting problem in that what you ask can be proved neither wrong nor true
    – sukhmel
    Aug 2 at 8:15
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I mean theoretically yes. Practically it gets rather complicated. Because that would mean that you'd also have to doubt your own emotions and I don't mean in the sense of optical illusions, which are interpretations of perceptions but perception itself.

So good luck trying to doubt you stubbed your toe or that you had a paper cut. Your body is probably going to tell you that it is an "undoubtable" fact.

But suppose you manage to drive yourself insane and doubt whether hunger and sleep deprivation are even real. Then you're rather sooner than later going to end up being dead and I bet you can't doubt that. (For the simple fact that you'd no longer be present to doubt that).

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  • "Let go the things of which you are in doubt, for the things in which there is no doubt." - Mohammed
    – Scott Rowe
    Jul 28 at 10:01
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We may be able to know one thing; that 'We know at least one thing'.

If we claim that 'We know nothing', we find that this can't be true, because the statement, if true, would mean we know at least one thing. And if it is not true, we must know something.

If we claim that 'We don't know that we know nothing', we again concede that if the statement is true, that we know at least one thing.

Even if one claims, 'I don't know which of the above statements is true', we again find ourselves claiming to know at least this.

The question of doubt however is different. Doubt can exist whether or not there is sufficient reason to doubt.

Therefore, it is quite possible to doubt every claim in this answer, including the claim that 'We know at least one thing', without changing the fact that we do know at least one thing.

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I don't know if you can do it but I can doubt it ;)

This is partly same as @philosodad's answer. Scepticism and the famous Cogito, ergo sum of René Descartes.

What is different from his answer is that at present times Yogis teach that the world is an illusion and being individual separate beings is also an illusion. Instead everything is One.

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I refer to an answer I made to a similar question:


There's two types of knowledge: Knowledge from looking at the universe, and created knowledge.

To be skeptical of created knowledge makes much less sense than knowledge derived from observation.

An example of created knowledge is the author of a story: How does he know where the hero has hidden secret widget? Because he decided.

Another example might be where you live: How do you know you live at 123 Acme Street? Because you decided to live there. I guess this latter combines observed knowledge that a house existed there in the first place, and an agent put a contract under your nose. But still there is a decision point there.

I guess it is possible to doubt your own decisions, but that route insanity lies, and is a common theme in psychological horror, such as Shutter Island.

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Yes, we can doubt reason.

Suppose you believe that the world is as it appears to your consciousness -- people walking around, air, earth, sky, sun, etc.

There are people in this world whose reasoning is very flawed. Their ability to reason appears very damaged. Sometimes you can even find a physical cause of that damage.

Your own chain of reasoning in your head could very easily be damaged in a similar way.

The classic "brain in a jar" problem, where you are nothing but a brain in a jar in some mad scientist's lab, all of your senses fed to you from a simulation, can be extended -- the mad scientist could be manipulating your internal reasoning.

It may be that 1 plus 1 is quite clearly 3, but the mad scientist which is poking at your reasoning is making you miss that clear fact every time you try to think about it.

Now, the idea that "we can doubt all reason" means we have to doubt the reasoning that leads us to doubt reason appears to be self defeating. Because either that logic is sound and correct, or it isn't; in both cases, doubting reason is right! However, that itself is an argument from reason, leading to infinite descent.

As a brain in a jar, we could have been assembled 5 seconds ago with false knowledge imprinted into it. Our memories and proven certainties could all be fabricated. What more, it is possible you are going to be assembled in 30 seconds, and this experience you are having is nothing more than a ghost of a memory your future self will experience.

Doubting something, however, doesn't mean you can't act on it.

I can doubt that some water is safe to drink and still drink it, and even be rational about doing so, because the alternative (to me) is worse. Even after drinking it, I can continue to have doubts about its safety.

You can act on information you doubt, on knowledge you doubt, and on chains of logic you doubt.

You can act in the absence of certainty. You can act even if certainty is doomed to be impossible. You can use your doubt to evaluate your choices and knowledge and logic and everything as you continue to rely on whatever tools you can get your hands on.

Now, you could also become sessile and cease all motion; there are people who go catatonic, sometimes for ways that appear to be psychiatric causes. But this too is a decision -- the decision of doing nothing.

0

I'll take a slightly different tack on this. Most knowledge builds upon previous knowledge, and so if you were to find fault with the previous knowledge, then everything that resulted from it would also be at risk.

By way of example, I know I have a mass of about 80kg. However, for me to know this, someone else has to know how to engineer a set of bathroom scales, and has to know how to calibrate it so that it counts the number of KG - a KG being defined by a lump of metal somewhere.

Further, for me to know my mass, someone else has to know that mass is independent of gravity, and can be measured using scales, whereas weight is measured differently. On Earth we can approximate mass to weight because we have "1G" of gravity. On the moon, we'd have to do some calculations to get from mass to weight.

Keep going with this line of thinking and you'll eventually wind all the way back to the very first bit of knowledge ever learned (this sort of mental journey is probably as easy as finding "the most funny joke ever", but by all means give it a try). Lets say the very first bit of knowledge is that the ground is hard.

I know the ground is hard because I can touch it, and it feels hard to me. However, if you were somehow able to prove that to be false, then pretty much all other human knowledge might also be wrong. A few things immediately spring to mind:

  • A glass will break if dropped onto a stone floor
  • the average walking speed of a human is about 3-4 miles per hour
  • it is more comfy sleeping on a bed than on the floor

...and so it goes on. You'd be able to falsify any number of things we think we know.

Your difficulty is of course to disprove these "core" pieces of knowledge. Working then on the assumption that you can't disprove them, then you can't immediately disprove all the things that depend on them. That's not to say you can't disprove any of them - just not as easily as if you could disprove the "core" things. Your quest is now splintered, rather than a single enquiry.

By now you're realising that some things are just really, really hard to disprove - and so aren't worth doubting. Other things are just really hard, but there may be more "wiggle room". For example, the average walking speed of a human might increase as we are tending to get taller (or maybe it'll reduce as we are also tending to get fatter?). The point being that there may be grounds to doubt such a fact - and you'll then go on to examine those areas of potential doubt and will then classify the facts to which they relate as either "too hard to disprove, so I don't doubt it", or "worthy of further investigation".

In summary then, yes you could doubt all knowledge, but in practice you'll find it to incredibly difficult to doubt some things and the best you might achieve is to doubt a fraction of all knowledge. How big that fraction is depends on what you're able to prove or disprove, and what other knowledge depends on it, I guess.

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