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[Epistemological scepticism is brilliant because it has a logical foundation that only clever people would be able to discover or understand, but it’s silly because that logical foundation is irrelevant—or, at least, Bertrand Russell thinks it’s irrelevant.]

Epistemological scepticism has a logical foundation, namely the principle that it is never possible to deduce the existence of something from the existence of something else. This principle must be stated more clearly, and without the use of the word “existence.” Let us take an illustration. You look out of the window, and observe that you can see three houses. You turn back into the room and say “three houses are visible from the window.” The kind of sceptic that I have in mind would say “you mean three houses were visible.” You would reply “but they can’t have vanished in this little moment.” You might look again and say “yes, there they are still.” The sceptic would retort: “I grant that when you looked again they were there again, but what makes you think they had been there in the interval?” You would only be able to say “because I see them whenever I look.” The sceptic would say “then you ought to infer that they are caused by your looking.” You will never succeed in getting any evidence against this view, because you can’t find out what the houses look like when no one is looking at them.

Our logical principle may be stated as follows: “no proposition about what occurs in one part of space-time logically implies any proposition about what occurs in another part of space-time.” If the reference to space-time is thought unduly suggestive of physicalism, it can easily be eliminated. We may say: “the perceptive propositions derivable from one perceived event never logically imply any proposition about any other event.” I do not think this can be questioned by any one who understands the logic of truth-functions.

But outside pure mathematics the important kinds of inference are not logical; they are analogical and inductive. Now the kind of partial sceptic whom we have been having in mind allows such inferences, for he accepts physicalism whenever it enables us to prophesy our own future percepts. He will allow the man measuring the velocity of sound to say “in five seconds I shall see the flag wave”; he will only not allow him to say “in five seconds the flag will wave.” These two inferences, however, are exactly on a level as regards induction and analogy, without which science, however interpreted, becomes impossible. Our logical foundation thus becomes irrelevant, and we have to consider whether induction and analogy can ever make it probable that there are unperceived events.

Bertrand Russell (from An Inquiry into Meaning and Truth, 1940)

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