Here is one hand  

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Here is one hand is the name of a philosophical argument created by George Edward Moore against philosophical skepticism and in support of common sense.

The argument takes the form:

• Here is one hand,
• And here is another.
• There are at least two external objects in the world.
• Therefore an external world exists.

Introduction

G. E. Moore (1873—1958) wrote A Defence of Common Sense and Proof of an External World. He posed skeptical hypotheses, such as "you may be dreaming" or "the world is 5 minutes old", creating a situation where it is not possible to know that anything in the world exists. These hypotheses take the following form:

The skeptical argument

Where S is a subject, sp is a skeptical possibility, such as the brain in a vat hypothesis, and q is a knowledge claim about the world:

• If S doesn't know that not-sp, then S doesn't know that q
• S doesn't know that not-sp
• Therefore, S doesn't know that q

Moore's response

• If S doesn't know that not-sp, then S doesn't know that q
• S knows that q
• Therefore, S knows that not-sp

Moore does not attack the skeptical premise; instead, he reverses the argument from being in the form of modus ponens to modus tollens. This logical maneuver is often called a G. E. Moore shift or a Moorean shift.

Explanation

Moore famously put the point into dramatic relief with his 1939 essay Proof of an External World, in which he gave a common sense argument against skepticism by raising his right hand and saying "here is one hand," and then raising his left and saying "and here is another". Here, Moore is taking his knowledge claim (q) to be that he has two hands, and without rejecting the skeptic's premise, proves that we can know the skeptical possibility (sp) to be not true.

Moore's argument is not simply a flippant response to the skeptic. Moore gives in Proof of an External World, three requirements for a good proof. (1) the premises must be different from the conclusion, (2) the premises must be demonstrated, and (3) the conclusion must follow from the premises. He claims that his proof of an external world meets those three criteria.

In his 1925 essay A Defence of Common Sense he argued against idealism and skepticism toward the external world on the grounds that they could not give reasons to accept their metaphysical premises that were more plausible than the reasons we have to accept the common sense claims about our knowledge of the world that skeptics and idealists must deny. In other words, he is more willing to believe that he has a hand than believe the premises of a strange argument in a university classroom. "I do not think it is rational to be as certain of any one of these ... propositions".

Not surprisingly, those inclined to skeptical doubts often found Moore's method of argument not entirely convincing. Moore, however, defends his argument on the surprisingly simple grounds that skeptical arguments seem invariably to require an appeal to "philosophical intuitions" that we have considerably less reason to accept than we have for the common sense claims that they supposedly refute.

Logical form

The skeptical argument takes the form of modus ponens:

• If A then B.
• A.
• Therefore B.

Moore's argument flips the modus ponens structure into a modus tollens:

• If A then B.
• Not B.
• Therefore not A.

This illustrates Fred Dretske's aphorism that "[o]ne man's modus ponens is another man's modus tollens"

Effect

Appeals of this type are subsequently often called "Moorean facts". "A Moorean fact [is] one of those things that we know better than we know the premises of any philosophical argument to the contrary".

The "here is one hand" idea, in addition to fueling Moore's own work, deeply influenced Ludwig Wittgenstein, whose last writings were devoted to a new approach to Moore's argument. These remarks were published posthumously as On Certainty.

See also

• Samuel Johnson, to whom is attributed the act of hitting a rock with his foot as a "refutation" of immaterialism