4.2 Possible Answers to Scepticism of the External World

What this lecture covers

Following the discussion of Descartes and the Cartesian circle, this lecture looks at G.E. Moore's famous "here is one hand" response to external-world skepticism. Millican shows why the argument is more sophisticated than it first appears, using it to illustrate a general point about how philosophical arguments can be run forward or backward.

You'll come away able to explain Moore's argument, distinguish modus ponens from modus tollens, and see why many philosophers turned toward externalist approaches instead of Cartesian ones.

Key ideas

• Moore's argument: "here is one hand, here is another; hands are physical objects; therefore an external world exists."
• Modus ponens: the valid inference form "P implies Q, P is true, therefore Q is true."
• Modus tollens: the equally valid inference form "P implies Q, Q is false, therefore P is false"—used to argue backward from a conclusion to reject a premise.
• Arguing forward vs backward: given an implication, you can either accept the premise and infer the conclusion, or reject the conclusion and infer the premise is false; which move you make depends on which claim you find more plausible.
• Applying this to Moore: Descartes's approach argues that since we don't know there's an external world, we don't know this is a hand; Moore argues the reverse—since we know this is a hand, we know there's an external world.
• Weighing plausibility: Moore's case rests on the claim that "we know this is a hand" is more credible than the skeptic's claim that "we don't know there's an external world."
• A shift to externalism: because Cartesian arguments that try to validate our faculties from the inside run into circularity, many philosophers moved toward externalist theories of knowledge, previewed here for the next lecture.

Before you watch

• Watch the previous lecture on Descartes and the Cartesian circle, since Moore's argument is presented as a response to the difficulty that lecture raises.

Check your understanding

1. How does Moore's argument reverse the direction of the skeptic's reasoning?
2. What is the difference between modus ponens and modus tollens, and how does Millican use both to analyze Moore's argument?
3. Why might someone find "we know this is a hand" more plausible than "we don't know there's an external world"?
4. Why do Cartesian attempts to validate our faculties "from the inside" struggle to escape circularity?