1. The Problem of the External World Kareem Khalifa Philosophy Department Middlebury College

2. Overview I.A specific argument for skepticism II.Generalizing the argument III.Why the argument is tough How can we have any knowledge of the external world?

4. II. Generalizing the Argument – External world is anything that exists independently of our experiences and mental states (e.g. rocks.) – Skepticism is only a claim about what we can know about the external world. – It’s not a claim about what exists in the external world.

5. II. The General Argument Let o be some ordinary proposition about the external world, and let h be some skeptical hypothesis. P1.If you know that o, then you know that not-h. P2.You don’t know that not-h. SC.So, you don’t know that o. In other words, you know nothing about the external world! Also: Write this down! In-Class Exercise

6. III. Why this argument is tough… A.Its logical structure B.Its scope

7. III.A. Logical Structure Both premises, P1 and P2, are very hard to deny. – They work with very ordinary examples about knowledge. But if P1 and P2 are true, then the skeptical conclusion, SC, has to be true.

8. III.B. Scope of skeptical argument This covers all knowledge of the external world. So, any further evidence that you might use to argue that you’re not dreaming is going to run into the same problem.

9. Example Suppose that there is a brain scan that indicates when you’re not dreaming. But how do you know that you’re not dreaming about that brain scan?

10. Recap The skeptic’s argument is simple, and rests on very plausible premises. That makes it tough to rebut.