I am thinking that simple statement such as "I am", "I think" are all beliefs that are also knowledge and conditional statements such as "This dog does exist in this world if the world is real" are also knowledge since they are true even if proven to be an illusion.

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    No, and it makes little sense. To a radical skeptic every statement is in doubt, specific statements cannot be counterexamples to an attitude. Especially one that applies to all statements, them included.
    – Conifold
    May 22, 2021 at 23:46
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    "To a radical skeptic every statement is in doubt..." I hope you're ready to back that up! ;) May 24, 2021 at 14:42
  • "are also knowledge since they are true even if" - I think "knowledge" is about more than just statements that are necessarily true. The statement also probably needs to actually tell you something about the world, which a conditional statement arguably doesn't do. The example given might also be problematic because you might be imagining the dog even if the world is real. Even if you assume the world is real, there is still very little you can say for sure if you're skeptical of your perception of it.
    – NotThatGuy
    May 24, 2021 at 15:26

4 Answers 4


A good question.

Every machine is fallible, including the human brain; this means that when the human brain comes up with any conclusion, there is always the possibility it made an error and came to the wrong conclusion. Even if the brain in question is your own. Computers can make occasional errors in calculation due to manufacturing defects, cosmic ray impacts, hard disk drive failure, software bugs, or even quantum tunneling (the latter with extremely low, but nonzero probability, and the fact it is nonzero is all that matters to the point). That means that when a computer tells you the result of any calculation, you cannot rationally trust it 100%. Perhaps you can trust it very close to 100%, but never exactly 100%.

The human brain is a piece of hardware subject to many of the same kinds of faults that affect computers. Humans are known to make calculation errors at a much higher rate than computers do. Therefore, we should not say we "know" something, unless by that we mean that we believe it with high confidence. Knowledge is a kind of belief, not something fundamentally different from belief. Everything is in doubt.

"Even that?"

Yes. It's a consequence of the physical basis of your brain.

Note that, if everything is uncertain, the degree of uncertainty is also uncertain. If you calculate a 99.999999% chance that you're right about X, you can't trust that calculation perfectly. Maybe you made an arithmetic error and really the chance of being right about X is only 30%!

As a practical matter, almost everyone is more confident of their own views than is objectively warranted. People, probably including you and me, would be more rational if they were less certain. Doubting your views is the only way to change them, and changing your views is the only way to become more right over time. Of course, it is not guaranteed that change is for the better; doubt also the change. But because like most people you likely have a bias towards overconfidence in your established views, it is necessary to put more effort into doubting your own views than doubting the change.

I've sometimes seen a strange reaction to the fact that all knowledge is uncertain. Some people associate uncertainty with the notion that truth is subjective, or that because we can never certainly reach absolute truth, it doesn't matter to us. This is like saying that because maps are sometimes printed with mistakes, mountains don't exist or don't matter to us. It's a complete non sequitur. It is entirely possible to both believe that a mountain exists because it's shown on the map, and that the mountain might not exist because the map might be misprinted. Not only is it possible, it's the most reasonable perspective. One believes in objective reality at the same time as one acknowledges that all of one's beliefs may be mistaken.


One ancient counterexample for radical skepticism is just ask such a skeptic whether he or she accepts the skeptic's own knowledge that "one should reject any knowledge". If the skeptic accepts then he or she contradicts own POV literally, if one rejects then one rejects one's own POV directly. Either result seems logically awkward for the radical skeptic.

Some modern radical skeptics use Brain in a vat (BIV) to argue that technology can be so perfect that one can never know if one is real or just a "disembodied" brain in vat, thus knowledge is impossible:

The simplest use of brain-in-a-vat scenarios is as an argument for philosophical skepticism and solipsism. A simple version of this runs as follows: Since the brain in a vat gives and receives exactly the same impulses as it would if it were in a skull, and since these are its only way of interacting with its environment, then it is not possible to tell, from the perspective of that brain, whether it is in a skull or a vat. Yet in the first case, most of the person's beliefs may be true (if they believe, say, that they are walking down the street, or eating ice-cream); in the latter case, their beliefs are false. Since the argument says one cannot know whether one is a brain in a vat, then one cannot know whether most of one's beliefs might be completely false. Since, in principle, it is impossible to rule out oneself being a brain in a vat, there cannot be good grounds for believing any of the things one believes...

Please note that any asserted knowledge like "I have a hand" could be leveraged by a radical skeptic as an argument for radical skepticism holding the view that one cannot know she is a BIV or not since technology may be prefect enough not to be able to distinguish any more. How? First the skeptic will claim we all agree with a self-evident principle of epistemic closure:

Epistemic closure is a property of some belief systems. It is the principle that if a subject S knows a (proposition) p, and S knows that p entails q, then S can thereby come to know q.

Since the proposition you have a hand entails the proposition you are not a BIV, once you know you have a hand, then you must also know you are not a BIV. But this is a contradiction with the undisputable fact that one can never know if one is BIV or not as introduced context above. So you cannot claim you know you have a hand...

Then how to construct a counterexample for this hopeless situation? Some philosophers like Robert Nozick proposed we have to abandon above "self-evident" principle of epistemic closure and change the philosophical JTB definition of knowledge. If you adopt this approach, then by external experimental confirmation of the existence of our hands then we can somehow confidently claim we have some certain real knowledge if common probabilistic types of knowledge doesn't qualify per your standard...

  • 5
    One should doubt any knowledge, not reject it. And yes, one should doubt the idea that one should doubt all knowledge. There is no contradiction here, just a surplus of uncertainty.
    – causative
    May 23, 2021 at 0:49
  • IMO the brain-in-a-vat thing doesn't go far enough, because it doesn't extend to doubting mathematical knowledge, which we also should do. It is better instead to look at how fallible the process of thinking itself is, such that we cannot ever completely trust that what seems sound reasoning truly is so.
    – causative
    May 23, 2021 at 1:06
  • @causative For a thorough empiricist one may doubt math since everything is synthetic a posteriori by deviating from Kant... A radical skeptic not only healthily doubt any knowledge, he or she may completely reject any knowledge outright... May 23, 2021 at 1:22
  • We may accept that we have a priori knowledge, interpreting it as knowledge encoded directly in our genes, before any experience. And we may still doubt it; just because our genes "tell" us something, doesn't mean they are correct. Many cognitive biases are apparently the result of our genes; they are adaptive, and a priori, but not correct. For the other point, I do not think it is rational to completely reject all knowledge, and that's not part of the description of radical skepticism here: en.wikipedia.org/wiki/Radical_skepticism
    – causative
    May 23, 2021 at 1:31
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    It can be used as an objection, but this objection is not about negating them and saying they are false - it is saying they are uncertain. You get a self-referential paradox from saying everything is false, but you don't get a paradox from saying everything is uncertain.
    – causative
    May 23, 2021 at 2:50

A realist and recovering ex-engineer weighs in.

The process of living one's life and (metaphorically) getting from A to B constitutes the solution of a never-ending series of problems. Engineer are trained to solve certain classes of problems in the physical world and with the passage of time and experience, they eventually get good at it and are able to pay bills, buy houses, support families and so on.

This process is imperfect, meaning you make mistakes and must recover from them and with luck converge on the truth through an iterative process which is built on the accumulation of facts and data.

As such there is no place for radical skepticism in this paradigm. Passenger planes, cars, houses, computers, bridges, freeways, MRI machines, sewage treatment plants and so on are not invented nor are they built by radical skeptics.

In this sense, therefore, radical skepticism is amply refuted by the ability of humans to solve problems, which has furnished us with the world we currently live in.

It is an imperfect world to be sure, but as a very recent cancer survivor I owe my current existence to MRI and CT machines and the people who know how to use them to solve problems.

  • I think the point of skepticism is that we can never be sure that we know anything. Sure, MRI and CT machines are great, but we don't really know how they work. Our current explanation is based on particles and fields which behave a certain way in space and time, but even most physicists admit they don't really understand quantum mechanics and relativity. Any day a new Einstein could come along and figure out that all our current beliefs about physics are actually wrong... May 23, 2021 at 16:33
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    @jcsahnwaldtReinstateMonica, The people who manufacture them do. Note that neither relativity nor quantum mechanics are needed to explain their operation. Note also that it has been known for decades that neither relativity nor quantum mechanics can be the ultimate explanation for all physical systems, in particular those which existed (only) in the very early universe. Finally, remember that whatever comes after GR and QM will in no way invalidate them but rather replace them in those special cases where they are already known to fail. V isit the physics SE for more about this. -NN May 23, 2021 at 19:14
  • "neither relativity nor quantum mechanics are needed to explain their operation" - That's wrong. We don't need QM for superficial explanations (e.g. "here's an image of your brain"), but of course nuclear magnetic resonance cannot be explained without QM. "GR and QM will in no way invalidate them" - Depends on what you mean by "invalidate". Newton's and Maxwell's theories are still useful approximations in many cases, and yet they have been "invalidated" - in many cases, their predictions are simply incorrect. May 23, 2021 at 22:18
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    P.S.: I just read my first comment here again and realized it's indistinguishable from some utterances by postmodernists, esotericists etc., who may say stuff like "our knowledge of science/astronomy/MRI machines isn't 100% reliable; therefore, religion/astrology/Reiki is just as valid". Of course, that's nonsense. See the last paragraph of @causative's excellent answer for details. May 24, 2021 at 11:21
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    The stories here, about the mistaken but operable theories of Niels Bohr and Arnold Sommerfeld, show that it would be possible to design a machine that could operably harness some natural phenomenon even though the designer actually misconceived that phenomenon. May 25, 2021 at 10:50

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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