Seyed Masoud Hosseini · Overview · Study log · Weekly summaries · Ideas · Search · Transcript · RSS feed

General Philosophy · Lecture 15 of 33 · 9:08

4.2 Possible Answers to Scepticism of the External World

Study guide

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

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

Vocabulary

external-world skepticism (noun): Doubt about whether the physical world outside the mind really exists.
Moore responds directly to external-world skepticism.
modus ponens (noun): A valid logical form: if P implies Q, and P is true, then Q is true.
Moore's argument can be read as a modus ponens.
modus tollens (noun): A valid logical form: if P implies Q, and Q is false, then P is false.
The skeptic can use modus tollens to reject Moore's premise.
implication (noun): A logical relationship where one statement, if true, guarantees another.
The argument depends on the implication between the two claims.
premise (noun): A statement assumed or claimed as true, used to support a conclusion.
Moore's premise is that this is a hand.
plausible (adjective): Reasonable to believe, seeming likely to be true.
Moore argues his premise is more plausible than the skeptic's.
credible (adjective): Able to be believed or trusted.
Moore's claim seems more credible than radical doubt.
externalism (noun): A theory of knowledge that locates justification outside the thinker's own mind, in the world or in reliable processes.
Externalism offers an alternative to the Cartesian approach.
sophisticated (adjective): Complex and cleverly developed, more than it first appears.
Moore's argument is more sophisticated than it looks.
reverse (direction) (verb): To turn something around so it goes the opposite way.
Moore reverses the direction of the skeptic's argument.
famous (argument) (adjective): Widely known and often discussed.
Moore's hand argument is famous in philosophy.
run forward (phrase): To follow an argument from premise to conclusion in the normal direction.
You can run the implication forward to reach the conclusion.
run backward (phrase): To follow an argument from a rejected conclusion back to reject a premise.
You can also run the implication backward using modus tollens.
weigh (plausibility) (verb): To judge and compare how believable different claims are.
We weigh plausibility to decide which claim to accept.
struggle (to escape) (verb): To have great difficulty achieving something.
Cartesian arguments struggle to escape circularity.
physical object (phrase): A material thing that exists in the external world.
Hands are physical objects, according to Moore.
valid (inference) (adjective): Logically correct, so the conclusion truly follows from the premises.
Modus ponens is a valid inference form.
shift (an approach) (noun): A change from one method or way of thinking to another.
There was a shift toward externalist theories of knowledge.
picked up (from) (phrase): Continued or resumed from a previous point.
This lecture picks up from the Cartesian circle.
preview (verb): To give a brief look at something that will be covered later.
The lecture previews externalism for next time.

From the YouTube description

A series of lectures delivered by Peter Millican to first-year philosophy students at the University of Oxford. The lectures comprise the 8-week General Philosophy course and were delivered in late 2009.

Slides for all his lectures can be found here:
http://podcasts.ox.ac.uk/people/peter-millican

Learn more about Oxford:
http://www.ox.ac.uk

← 4.1 Scepticism of the External World · 4.3 Introduction to Cartesian Dualism →