Chapter 5 primarily deals with radical external world skepticism. An interesting philosophical problem related to external world skepticism is called, “The Problem of Other Minds.” It goes as follows:

I know that I have a mind, as I am immediately and directly aware of my own consciousness and the contents of my own mind. I know my own mind immediately and directly. I am always in “direct” contact with my own mind. However, I never experience any other mind. I infer that other people have minds based upon their behavior. When someone talks or acts a certain way I simply take it for granted that the other has a mind and is in a mental state. If someone cries out and grabs her foot, I take it that she is “in pain” (which is a particular mental state). But I am never directly aware of any mind outside of my own. It might be the case that there are no other minds at all, and that I am being tricked by a cleaver robot that is programed to “act” like it as a mind. How could I ever be certain that anyone else has a mind? What, exactly, justifies a belief that other minds exist? Justifying that I have a mind is easy. I am immediately and directly aware of my own mind. But justifying the claim that someone else has a mind is quite different. All that I am ever acquainted with is behavior, which could be produced by something that does not really have a mind. Behave alone does not prove conclusively that a thing has a mind. A sophisticated robot could be programed to act like it has a mind. What, then, justifies the belief that other minds exist? How could I distinguish between (1) a thing that looks and behaves like it has a mind but does not–and (2) a thing that behaves like it has a mind because it does (in deed) have a mind??