When Is True Belief Knowledge? by Foley Richard

Author:Foley, Richard
Language: eng
Format: epub
ISBN: 9781400842308
Publisher: Princeton University Press

Chapter 15

Closure and Skepticism

Acknowledging that luck is not incompatible with knowledge makes it easier to deal with other puzzles as well.

Consider a variation of the barn story. The story begins like the original: George is touring farm country and is charmed by the old barns he is seeing; he pulls his car over to the side of the road to appreciate the latest he has happened across; and, as he looks out the window, he has a true belief that he is looking at a barn. In this version of the story, however, there are no barn facades in the region. During his tour until now, he has been seeing nothing but real barns. Moreover, he correctly believes that he has normal vision, there is nothing obstructing his view, and the light is good. He thus apparently knows that he is seeing a barn.

Suppose he now recalls the brain-in-a-vat hypothesis, according to which his brain is in a vat in a laboratory where it is being stimulated to produce precisely the sorts of experiences he is now having. He realizes the proposition that he is now looking at a barn entails that he is not a brain-in-a-vat. He accordingly believes this latter proposition, but does he know it?

Well, why not? His belief that he is not a brain-in-a-vat, like his belief that he is looking at a barn, is true, and as the story has been told, there are no obviously important gaps in his information about the situation.

On the other hand, to the degree that one regards it as questionable whether George can know that he is not a brain-in-a-vat, one might be inclined to think that the argument here has to be reversed, with the unwelcome inference being that George cannot even know that he is now seeing a barn.

The basis for such an inference is the principle that knowledge, to use the term of art, is closed under known entailment. According to this principle, if S is aware that A entails B, then S knows A only if she knows B. This is sometimes referred to as “single premise closure,” since it applies only to entailments from a single known proposition. A much stronger and correspondingly less plausible principle is that knowledge is closed under multiple premises: if S is aware that (A and B) entails C, then S knows A and knows B only if she knows C. Such a principle, for example, implies that in the preface S cannot know each of individual claims in her book without knowing the conjunction and hence without knowing that she has made no mistakes anywhere in the book. The above argument, however, requires only single premise closure along with the assumption that George does not know that he is not a brain-in-a-vat.1

But why make this latter assumption? George’s belief that he is not a brain-in-a-vat is true, and as far as we can tell there are no important truths about his circumstances of which he is unaware. So,

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