Here is one hand

Based on Wikipedia: Here is one hand

In 1939, the British philosopher G. E. Moore stood before an audience at the British Academy and did something remarkable. He raised his right hand and said, "Here is one hand." Then he raised his left and said, "And here is another." He paused. "Therefore, an external world exists."

That's it. That was his proof.

To understand why this simple gesture became one of the most discussed arguments in twentieth-century philosophy, you need to understand the problem Moore was trying to solve—and why his solution infuriated and fascinated philosophers in equal measure.

The Skeptic's Trap

Philosophical skepticism about the external world is an ancient worry that goes something like this: How do you know that anything outside your mind actually exists? You might be dreaming right now. You might be a disembodied soul experiencing elaborate hallucinations. You might be—to use a more modern version—a brain floating in a vat, hooked up to a supercomputer that feeds you false sensory experiences.

The French philosopher René Descartes imagined an "evil demon" with godlike powers who could create perfect illusions indistinguishable from reality. If such a demon existed, everything you think you know about the physical world—tables, chairs, other people, your own body—could be wrong.

The skeptic's argument has a disturbingly simple structure. Let's call it the Skeptical Syllogism:

If you can't rule out the possibility that you're being deceived by an evil demon (or that you're a brain in a vat), then you don't really know there are trees and mountains outside.

You can't rule out the possibility that you're being deceived.

Therefore, you don't really know there are trees and mountains outside.

Each step seems reasonable. The conclusion follows logically from the premises. And yet the conclusion is absurd—or is it?

For centuries, philosophers had tried to escape this trap by attacking the skeptic's premises directly, attempting to prove that we couldn't be deceived in such ways. Moore tried something different.

The Moorean Shift

Moore's strategy was elegant in its audacity. He simply flipped the argument around.

The skeptic argues: "If I can't prove I'm not a brain in a vat, then I don't know I have hands." Moore responds: "I do know I have hands—look, here they are!—therefore I can prove I'm not a brain in a vat."

This logical maneuver—taking your opponent's conclusion, negating it, and using that negation as a premise in your own argument—became known as the "G. E. Moore shift" or "Moorean shift." The philosopher Fred Dretske captured its essence in a pithy aphorism: "One man's modus ponens is another man's modus tollens."

To unpack that bit of jargon: modus ponens and modus tollens are two valid forms of logical argument. Modus ponens says "If A then B; A is true; therefore B is true." Modus tollens says "If A then B; B is false; therefore A is false." Moore noticed that the skeptic was using modus tollens (if you can't rule out deception, you don't know; you can't rule it out; therefore you don't know). He simply ran the same logic in reverse using modus ponens (if I know I have hands, I can rule out deception; I know I have hands; therefore I can rule out deception).

Both arguments are logically valid. So which one should we accept?

The Defense of Common Sense

Moore's answer, developed in his 1925 essay "A Defence of Common Sense," was that we should trust our ordinary beliefs about the world more than we trust the premises of any philosophical argument designed to undermine them.

Think about what the skeptic is actually asking you to believe. They're saying that because you can't disprove the possibility of an evil demon or a vat-maintaining supercomputer, you should doubt whether your hands exist—even while you're looking directly at them, touching them, wiggling your fingers.

Moore thought this got things exactly backwards. The evidence of his senses—the direct, immediate experience of having hands—was far more compelling than any abstract philosophical reasoning about hypothetical demons. When forced to choose between "I see my hands, therefore they exist" and "I can't disprove demons, therefore maybe my hands don't exist," he chose the former without hesitation.

This isn't mere stubbornness. Moore argued that the skeptic cannot provide reasons to doubt our hands that are stronger than the reasons we have to believe in them. We don't just suspect we have hands—we know it, with a certainty that no "strange argument in a university classroom" can shake.

These bedrock beliefs that resist philosophical assault became known as "Moorean facts"—things we know better than we know the premises of any philosophical argument to the contrary.

Moore's Proof and Its Criteria

In "Proof of an External World," Moore wasn't just gesturing at his hands to be dismissive. He was offering what he considered a rigorous proof, and he specified three criteria that any good proof must meet:

First, the premises must be different from the conclusion. You can't prove something by assuming it. Moore's premises—"here is one hand" and "here is another"—are indeed different from his conclusion that an external world exists.

Second, the premises must be known. Moore claimed he knew he had hands. He didn't merely believe it or hope it—he knew it, directly and immediately.

Third, the conclusion must follow from the premises. If hands exist as material objects external to the mind, then by definition something external to the mind exists. The logic is airtight.

Moore's critics have focused especially on that second criterion. Does he really know he has hands, or does he merely perceive that he has hands? There's a difference between having a perceptual experience and knowing that the experience corresponds to reality. Perhaps Moore was demonstrating perception when he needed to demonstrate knowledge.

Wittgenstein's Final Puzzle

Moore's argument "long interested" the philosopher Ludwig Wittgenstein—one of the most brilliant and enigmatic thinkers of the twentieth century. In fact, Wittgenstein spent the last six weeks of his life, in early 1951, working on a response to Moore's proof. It was the fourth time in two years he had tackled the problem. His notes from these attempts were published posthumously as "On Certainty" in 1969.

Wittgenstein's objection was characteristically subtle. He focused on the phrase "I know."

When we say "I know" in ordinary life, we're playing a particular language-game. We say "I know where the keys are" or "I know how to ride a bike" or "I know that Paris is the capital of France." In these contexts, "I know" is perfectly natural and unassuming.

But Moore was using "I know" in an unusual way. He was saying "I know that my hands exist" as a response to the possibility that reality itself might be an illusion. Wittgenstein suggested that this displaces "I know" from its proper context and produces a kind of philosophical confusion—not exactly a fallacy in the logical sense, but a misuse of language that generates apparent problems where none exist.

Even if one proposition logically implies another, Wittgenstein noted, knowing the first doesn't automatically mean knowing the second. The word "know" behaves differently in different contexts. Moore had borrowed a word from ordinary language and tried to make it do philosophical heavy lifting it wasn't designed for.

The Transmission Problem

In recent decades, the debate over Moore's proof has centered on a technical question: does the proof transmit justification?

Here's the issue. Suppose Moore's evidence for believing there's a hand is his visual experience of a hand. Everyone agrees that if there's a hand (a material object), then there's an external world (at least one material object exists). The logic is unassailable.

But the philosopher Crispin Wright argued that something goes wrong anyway. To use your visual experience of a hand as evidence that there's a hand, you must already be entitled to assume that your visual experiences generally correspond to reality—that is, that there's an external world. The very thing Moore is trying to prove is something he must assume in order for his proof to work.

Wright illustrated the problem with a thought experiment. Imagine twins named Jessica and Jocelyn who look absolutely identical. John sees a girl who looks like Jessica and reasons: "The girl in front of me is Jessica. If she's Jessica, she's not Jocelyn. Therefore, she's not Jocelyn."

The logic is valid. But can John actually use this argument to come to know that the girl isn't Jocelyn? His evidence that the girl is Jessica is that she looks like Jessica—but that's equally consistent with her being Jocelyn, the indistinguishable twin. His reasoning seems to go in a circle.

Wright claimed Moore's proof has the same problem. Moore's evidence for "there's a hand" is his visual experience. But his visual experience could be evidence for a hand only if he's already entitled to assume his experiences reflect reality. He can't use the proof to establish what he must already assume for the proof to work.

Liberals and Conservatives

The philosopher James Pryor disagreed with Wright, and their debate illuminates two fundamentally different ways of thinking about perception and knowledge.

Wright's view is what Pryor calls "perceptual conservatism." On this view, when you have an experience that seems to show you something—say, a hand in front of you—you can be justified in believing there's a hand only if you have independent reason to think you're not being deceived. Before you can trust your experiences, you need some prior justification for thinking your experiences are trustworthy.

The trouble with conservatism is that it's hard to see where this prior justification could come from. How do you rule out the evil demon without relying on your experiences? If you can't, conservatism leads directly to skepticism—the very problem Moore was trying to solve.

Pryor advocates what he calls "perceptual dogmatism." On this view, when you have an experience as if there's a hand in front of you, that experience immediately and directly justifies you in believing there's a hand. You don't need to rule out demons first. The experience itself provides justification, without requiring any prior assumptions.

If perceptual dogmatism is right, then Moore's proof works. His visual experience of a hand directly justifies him in believing there's a hand. From "there's a hand," he can validly conclude "there's an external world." The justification transmits through the argument, and Moore has indeed proved what he set out to prove.

Odd But Not Wrong

Even Pryor admits that Moore's proof seems "odd" somehow. But he argues the oddness is dialectical rather than epistemic.

What does that mean? The proof may actually succeed in providing justification for believing in an external world. But it won't convince a determined skeptic, because a skeptic already doubts the conclusion before examining the proof. Since the skeptic doubts there's an external world, she won't accept visual experience as evidence for hands. She'll say Moore is begging the question—assuming what he needs to prove.

Moore's proof might be perfectly good as a proof while being useless as a persuasive tool. It can justify belief for someone willing to trust their experiences, but it can't convert someone who isn't.

The philosopher Luca Moretti offered another explanation for the proof's oddness. If perceptual dogmatism is right, then when Moore sees a hand, his experience directly justifies both "there's a hand" and "there's something material" (since a hand is a material thing). He doesn't need to reason through the argument at all—he gets justification for the conclusion immediately from his experience. The proof is strange because it's actually superfluous. It adds nothing to what Moore already has just by looking at his hands.

A Long Tradition of Direct Refutation

Moore wasn't the first philosopher to respond to abstract skeptical arguments with concrete demonstrations.

When the Irish philosopher George Berkeley argued in the early 1700s that matter doesn't exist—that everything we perceive is purely mental—the English writer Samuel Johnson famously responded by kicking a large rock and declaring, "I refute it thus!" Johnson's foot hurt; therefore, the rock was real.

Even earlier, the ancient Greek philosopher Diogenes reportedly responded to a philosopher arguing that motion is impossible by simply getting up and walking away. No words were necessary. His movement was the refutation.

The Russian novelist Leo Tolstoy captured a similar sentiment in "War and Peace" when addressing determinism—the view that all our actions are completely caused by prior events and we have no genuine freedom. Tolstoy wrote: "You say: I am not free. But I have raised and lowered my arm. Everyone understands that this illogical answer is an irrefutable proof of freedom."

There's something deeply appealing about these responses. They cut through centuries of sophisticated argumentation with the brute force of immediate experience. And yet philosophers continue to debate whether they actually work, or whether they merely sidestep the real issues.

The Limits of Common Sense

Moore's defense of common sense raises a profound question: when should we trust our immediate intuitions over logical arguments, and when should we let arguments override intuitions?

Sometimes logic should win. Our intuitions tell us the Earth is flat and stationary, but science has shown otherwise. Our intuitions say an object in motion will naturally slow down, but Newtonian physics says it will continue forever unless something stops it. Common sense once said heavier objects fall faster than lighter ones—until Galileo showed otherwise.

Other times, intuition seems like a reasonable stopping point. If a philosophical argument concludes that I don't exist, or that the past never happened, or that nothing is moving, it seems sensible to reject the conclusion rather than accept it. Some conclusions are simply too absurd to believe, regardless of how clever the argument.

Moore's contribution was to take this intuition seriously and give it philosophical respectability. He didn't just grunt and kick a rock—he articulated why we might be entitled to trust common sense over skeptical arguments, and he tried to show that such trust could be part of a rigorous philosophical position rather than mere anti-intellectualism.

Whether he succeeded remains an open question. But his two raised hands continue to gesture at something important: the tension between what we think we know and what we can prove we know, between living in the world and theorizing about it.

Here is one hand. And here is another. What follows from that simple fact may be more complicated than it first appears—or simpler, depending on which philosopher you ask.