External world skepticism is the view that we cannot know anything about the external world: we can’t know that we have hands, that there are other people, or, in general, know that anything external to our minds exists.
Such skeptics commonly argue that we can’t refute skeptical hypotheses—such as that everything is a projection of our own minds—and that since we can’t do that, we can’t know that we have hands or much else.
British philosopher G. E. Moore (in)famously purported to refute external world skepticism with what’s known as Moore’s Proof: he held out his hands during a lecture and said “Here is one hand” and “here is another”, claiming to prove that there is an external world on that basis.
While philosophers tend not to think that Moore’s Proof refutes skepticism, it is hard to explain why. This essay reviews some of the most important attempts.
1. Transmission Failure
To begin evaluating Moore’s Proof, let’s consider a potentially analogous “proof”:
Imagine Sally and Susie are at an art exhibition viewing a wall that appears red. Sally asks, “What color is the wall?” Susie replies, “Well, the wall looks red.” Sally counters with, “But couldn’t the wall be white with hidden red lights shining on it?” Susie answers: “The wall looks red, and so it is red. Therefore, it’s not white with hidden red lights shining on it.”
Did Susie prove that there are no hidden red lights shining on the wall?
Intuitively not. But why?
Some philosophers have argued that the reason why responses like Susie’s fail is because it’s a case of what’s known as “transmission failure.” This occurs when someone’s belief A entails belief B (B is a logical consequence of A), but their support for A is not support for B, because prior support for B is necessary for getting support for A.
On this view, the reason why Susie didn’t prove that there are no hidden red lights is that, by relying on her perceptions to believe that the wall is red, she is presupposing that there are no such lights.
Some philosophers think that Moore’s “proof” is also a case of transmission failure. Moore believes that he has hands, and this entails that there’s an external world. But, intuitively, his perceptions could support his belief that he has hands only if he already has support for thinking there is an external world. For aren’t hands external objects?
2. Liberalism and Conservatism
However, some philosophers argue that Moore’s Proof is not a case of transmission failure. On their view, we don’t need evidence for his conclusion in order to have perceptual evidence for his premise. Instead, we only need to lack evidence against the conclusion in order for our perception as of our hands to support the premise that we have hands. This is called liberalism about perceptual justification:
Liberalism: For your perception to provide evidence for your external world beliefs, you must lack evidence for skeptical hypotheses.
In contrast, proponents of transmission failure seem committed to:
Conservatism: For your perception to provide evidence for your external world beliefs, you must already have evidence to reject skeptical hypotheses.
The trouble is that conservatism seems to imply skepticism. How could you get evidence against skeptical hypotheses if not by relying on your perception? This supports taking a closer look at liberalism.
One way of developing liberalism is dogmatism. According to dogmatism, if your perception as of something’s being the case (call it “P”) makes it seem to you that P, then provided that you lack sufficient evidence to doubt that P, you are justified in believing that P.
For example, right now your perceptions make it seem to you that you are reading an essay. Now consider whether you have evidence to believe that you are not reading an essay. (Do you?). Provided you lack such evidence, dogmatism implies that you are justified in believing that you are reading an essay.
The dogmatist extends this idea to Moore’s Proof as follows:
It seems to you that you have hands. You also lack evidence to believe that there is no external world. So, your perception justifies you in believing that you have hands. So, you are justified in believing in an external world.
Dogmatism by itself, however, doesn’t resolve our puzzle about why Moore’s ‘proof’ is unsatisfying. If I’m a skeptic and you rehearsed Moore’s Proof for me, I probably wouldn’t be persuaded. But why not? After all, according to dogmatism, I apparently have justification to believe the conclusion! That’s what we’ll explore next.
4. Why is Moore’s Proof Unpersuasive?
Dogmatists argue that Moore’s Proof might not persuade the external world skeptic but that this doesn’t make it a bad argument. This is because there is a difference between arguments which give us justification to believe their conclusions—justifying arguments—and arguments which persuade people who doubt the conclusion—persuasive arguments. An argument with unjustified premises might persuade someone to overcome their doubts about the conclusion. Likewise, an argument with justified premises might fail to persuade someone to believe a conclusion they have justification to believe.
The skeptic doubts that there is an external world, which undermines her confidence in her perceptions. Moore’s Proof won’t help her overcome her doubts. This is because her doubts prevent her from accepting the premise of Moore’s Proof (“I have hands”), even though her perceptions do give her justification to believe that premise.
Moore’s Proof starts from the assumption that one needn’t doubt their perceptions unless there is concrete evidence of unreliability, like bad weather or intoxication. The skeptical hypothesis that everything is a projection of one’s mind, however, is a mere logical possibility; there’s no concrete evidence that it might actually be the case. For this reason, it shouldn’t undermine one’s trust in their perceptions.
Most people who first encounter Moore’s Proof typically think that it’s a total failure. As we’ve seen, it’s not clear why or even whether it is a failure.
 Skeptical hypotheses are hypothetical scenarios in which your beliefs could systematically be false, but they would nevertheless appear true; everything would seem the same to you.
Descartes famously discusses two skeptical hypotheses: if we were dreaming or if were deceived by an evil demon, our beliefs about the external world would be false yet everything would appear “normal” to us. See Descartes’ Meditations 1-3 by Marc Bobro.
Zhuangzi raises the possibility of dreaming with a metaphysical twist; that we could be butterflies dreaming that we are human beings. See Zhuangzi (2013: 18) in Watson (ed.) (2013).
 Moore’s Proof is typically standardized as follows:
Premise: Here are two hands.
Therefore, conclusion: There is an external world.
See Wright (2002) for this way of standardizing the proof.
Here’s how Moore put his proof:
“I can prove now, for instance, that two human hands exist. How? By holding up my two hands, and saying, as I make a certain gesture with the right hand, ‘Here is one hand’, and adding, as I make a certain gesture with the left, ‘and here is another’. And if, by doing this, I have proved ipso facto the existence of external things, you will all see that I can also do it now in numbers of other ways: there is no need to multiply examples” (Moore 1993: 166).
 Compare with Barry Stroud (1984), who said that “we immediately feel that Moore’s Proof is inadequate” (1984: 86). Annalisa Coliva (2008) calls the proof “an obviously annoying failure” (Coliva 2008: 235). J. Adam Carter, in his (2012) survey of recent work on Moore’s Proof, says that the proof is “intellectual unsatisfying” (Carter 2012: 115).
 Wright (2002) defends this position. Wright (2004) further develops this into an anti-skeptical strategy.
 Some philosophers argue that we could have hands even if we were brains in vats living inside a simulated world. For them, ‘hands’ and other ordinary objects are defined in terms of what they do rather than what they are made of, whether it be organic matter or 0’s and 1’s. For this reason, some skeptical hypotheses are compatible with our ordinary beliefs. See Chalmers (2018) and Walker (2020).
 Liberals also tend to be Neo-Mooreans, who hold that Moore’s Proof is sound, and thereby that it really does prove that there’s an external world. Unlike Moore, however, they try to explain how we can know or get evidence to believe the premise ‘here are two hands’. See Pritchard (2007) for an introduction.
 See Pryor (2004), section 3, for this way of thinking about the relationship between perceptual evidence and skeptical hypotheses.
 Pryor (2000) argues for this claim. Conservatives typically reply by arguing that there is a species of justification for beliefs that doesn’t rely on having evidence, so-called entitlement, and argue that we can be entitled to believe that there’s an external world without having evidence for it. See Wright (2004). Some philosophers, like Coliva (2015), argue that it is constitutive of being a rational agent that you accept that there’s an external world.
 For a defense of dogmatism, see Pryor (2000). Dogmatism is an example of a more general thesis in epistemology called phenomenal conservativism, the idea that if it seems to you that P when you lack defeaters—reasons to think that P is false, or that you are making mistake—you thereby get some degree justification to believe P. See Huemer (2001) and Tucker (ed.) (2013).
 This is a simplified form of Pryor’s (2011), section 7, discussion of why some proofs with good justificatory structure, like Moore’s Proof, do not persuade skeptics. As Pryor puts it, if you believe without evidence that you are in a skeptical scenario, this belief would not undermine the evidence that your perceptions give you to believe that you have hands, but it will “rationally obstruct” you from believing that you have hands (Pryor 2011: 368).
External World Skepticism by Andrew Chapman
Descartes’ Meditations 1-3 by Marc Bobro
al-Ghazālī’s Dream Argument for Skepticism by John Ramsey
Possibility and Necessity: An Introduction to Modality by Andre Leo Rusavuk
Idealism Pt. 1: Berkeley’s Subjective Idealism by Addison Ellis
About the Author
Chris Ranalli is a lecturer in epistemology at the VU University Amsterdam and post-doctoral fellow for the Extreme Beliefs ERC project. He received his Ph.D. in Philosophy from the University of Edinburgh. He specializes mainly in epistemology, but he has interests in philosophy of mind and the intersection of epistemology and moral philosophy. He is currently exploring topics in social epistemology and the ethics of belief. chris-ranalli.weebly.com