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:
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- 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
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- 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.
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