How I Learned to Stop Worrying About The Existence of An External World

by Joseph Shieber

If you’re like me, when you read something on 3QD you often have a cup of coffee ready to hand. Perhaps you’re at your desk at work on your computer, with a mug near your mousepad. Or you’re in a coffee shop reading on your cell phone. Or at home reading on a tablet in your armchair with a cup perched on a side table conveniently within reach.

Without looking away from the screen, you’ll sometimes reach over for the cup, grab it, and bring it to your mouth to drink. This virtually automatic, quotidian activity is actually extremely complex. In order for you automatically to grab your coffee cup and take a sip, your brain has to keep track of where your hand is, where the coffee cup is, how to move your hand to the coffee cup, how to grasp the handle of the coffee cup, and then how to move your hand — now grasping the cup — to your lips. Yet we perform these simple actions virtually without thinking!

What I’d like to focus in on is the first step of that sequence of simple actions — knowing where your hand is. That’s because a recent paper by Kazumichi Matsuyima in Scientific Reports sheds new light on what it is that our brains do in order for us to know where the parts of our body are.

Thinking about this will also allow me to review one of the most famous proofs in twentieth century analytic philosophy. And it will allow me to talk a little about one of the most discussed results in recent cognitive science. All of which will give me a chance to talk about some cool stuff that I wasn’t able to fit into my lectures for The Great Courses that were released last month — Theories of Knowledge: How to Think About What You Know.

First, the philosophy proof, which can be found in G.E. Moore’s essay, “Proof of an External World”. It’s intended as a proof of the existence of, well, an external world: of the existence of things other than minds and mental states.

The proof itself is quite short. Here it is:

I can prove right 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’. (G.E. Moore, “Proof of an External World”)

Notoriously, there are a number of issues with the proof.

First of all, it’s not terribly original. Arguably, Samuel Johnson gave basically the same proof in response to Bishop Berkeley’s idealism, some 150 years before Moore’s proof:

After we came out of the church, we stood talking for some time together of Bishop Berkeley’s ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it – “I refute it thus.” (Boswell, Life of Johnson)

Second, there are many who question whether the proof can really accomplish its goal — assuming that the goal of the proof is to establish the existence of an external world to the satisfaction of someone who is prepared to doubt that existence.

Perhaps the most famous instance of this objection against Moore is Wittgenstein’s On Certainty. There, in response to Moore, Wittgenstein rejoins that

The statement “I know that here is a hand” may then be continued: “for it’s my hand that I’m looking at.” Then a reasonable man will not doubt that I know. – Nor will the idealist; rather he will say that he was not dealing with the practical doubt which is being dismissed, but there is a further doubt behind that one. (Wittgenstein, On Certainty, §19)


The idealist’s question would be something like: “What right have I not to doubt the existence of my hands?” (And to that the answer can’t be: I know that they exist.) (Wittgenstein, On Certainty, §24)

The worry that Wittgenstein is raising against Moore, then, is that Moore is begging the question against the person who genuinely wishes to doubt the existence of an external world. It’s not fair for Moore to adduce external-world evidence in order to prove the existence of an external world. To use an example that Wittgenstein employs in a different context, it would be like proving the accuracy of a newspaper article by buying a second copy of the same newspaper and comparing the two articles!

This second worry is a very deep one. I think that there is a good answer to it, but for now I want to look at a different issue raised by Moore’s proof.

Look at that first quote from Wittgenstein again. Wittgenstein seems to suggest that there is strong evidence — in normal contexts at least — for the claim, “Here’s a hand”, because as Wittgenstein puts it, “it’s my hand I’m looking at”!

This does seem to be the sort of reception that Moore was after when he chose the claim, “Here is one hand” for his proof. Moore writes:

I knew that there was one hand in the place indicated by combining a certain gesture with my first utterance of ‘here’ and that there was another in the different place indicated by combining a certain gesture with my second utterance of ‘here’. How absurd it would be to suggest that I did not know it, but only believed it, and that perhaps it was not the case! You might as well suggest that I do not know that I am now standing up and talking — that perhaps after all I’m not, and that it’s not quite certain that I am! (Moore, “Proof of an External World”)

What Wittgenstein and Moore seem to agree on, then, is that my awareness of my own hand — both where it is in space and that it is my hand — enjoys a special sort of certainty (again, in normal contexts). The implication seems to be that, in the language of philosophy, it is a foundational claim — a claim that can be used to justify other claims, but that is not itself in need of justification or support.

Moore’s discussion seems to conflate two different phenomena. One is justification: the reasons that a person herself would offer in favor of her beliefs, or that others would be willing to accept in favor of those beliefs. The other is support: the actual causes of a belief.

My own view about knowledge is that justification and support can come apart. You can have strong justification for a belief that is in fact not well-supported. And you can have strong support for a belief for which you have little justification. On my view, roughly, you have knowledge when you have a true belief that is well-supported, when the support for that belief makes it highly likely to be true. Justification doesn’t matter.

Now here’s where the empirical studies from cognitive science come in. They won’t go all the way to providing evidence for my own view. In particular, they won’t demonstrate that justification doesn’t matter. But they do seem to underscore the importance of support (rather than justification) and the ways that underlying brain structures that work below the level of awareness can profoundly influence what we believe.

In 1998, Matthew Botvinick and Jonathan Cohen published a brief note in the journal Nature describing a series of studies they’d done in which they’d induced their experimental subjects to “feel” as if a fake, rubber forearm and hand was actually the subject’s arm. Here’s how they did it.

They had the subjects sit behind a screen so that when they peered over the screen, what they saw was the fake, rubber forearm and hand rather than their own forearm and hand, which rested on a table parallel to the fake appendage. Then, they showed the subjects the fake arm being brushed gently with a paintbrush, while at the same time they brushed the subject’s actual, hidden forearm and hand, taking pains to synchronize the brushstrokes as closely as possible. And what the subjects reported was “feeling” the brushstrokes — not on their actual, hidden arm, but in the location of the fake, rubber arm! In other words, it “felt” as if the fake arm was their arm! (There’s a description of a similar experiment, with pictures, here.)

What this suggests to me is that your belief that your hand is your own — although it feels simple and foundational — is actually more complex, and based on more complex brain processes than it appeared to Moore.

For about two decades now, scientists have thought that the same processes that allow you to know where your body is in space are also the processes that allow you to feel ownership over your body. On this way of thinking, the reason why you can be tricked into thinking that you “own” the fake rubber hand in Botvinick and Cohen’s illusion is that your brain is tricked into thinking that your hand is located where the rubber hand is.

A new article by Kazumichi Matsumiya in Scientific Reports, however, complicates this picture even further. According to Matsumiya, “the neural substrates for perceptual identification of one’s body parts—such as body ownership—are distinct from those underlying spatial localization of the body parts, thus implying a functional distinction between ‘who’ and ‘where’ in the processing of body part information”.

If this new picture is correct, this would suggest that the brain processes underlying the sort of belief that Moore relied on for his proof of an external world are actually even more devilishly complex! It takes a lot more for me to believe, “Here is a hand” than Moore himself — or Wittgenstein, for that matter — ever imagined.

I said that I wouldn’t really address the worry that Moore’s argument begs the question against the person who is prepared to doubt the existence of an external world, but let me spend a moment discussing that worry, in lieu of a conclusion.

Suppose I’m right, and Moore was actually hasty in thinking that treating beliefs like “Here’s a hand” as foundationally justified was all there was to such beliefs. On my picture, what makes those beliefs count as knowledge, when they do count as knowledge, is that the underlying — and extremely complex — brain processes that result in those beliefs are working properly in appropriate environments.

What studies like Botvinick and Cohen’s — or, more recently, Matsumiya’s — demonstrate is the way in which such beliefs can go astray. They show ways in which we can believe “Here’s a hand” — that is, here’s my hand — without having the appropriate support. Such cases are ones in which we have the seemingly foundational, justified beliefs, but do not have knowledge. By demonstrating that it’s possible to have justified beliefs that aren’t well-supported, however, these studies underscore the importance of support for knowledge.

But for someone who’s actually willing to doubt the existence of an external world, referring to empirical research like that of Botvinick, Cohen, or Matsumiya in the context of a philosophical discussion about knowledge is just as much an instance of question-begging as Moore’s proof. If there’s no external world, then there are no rubber hands or other experimental apparatus either!

The unspoken implication of somebody who wants to press their doubts about the existence of an external world is that those doubts are the truly interesting ones, and that questions like those concerning how your brain comes to locate your limbs in space, or how it comes to see parts of the world as “belonging” to you are less interesting. This seems to me to be profoundly misguided.

So ultimately, my strategy for dealing with skepticism about the existence of the external world isn’t, as Moore does, to try to provide a refutation. Rather, it’s to attempt to change the subject.