Let us consider some case where we seem more certain of the truth of our general proposition, say “all dodos are mortal.” We know this, it may be said, because all dodos are dead. It might be objected that perhaps there are dodos in other planets, or that evolution, having produced the dodo once, may produce it again, and next time may make it immortal, like the phoenix. We will therefore amend our general proposition, and say only: “all dodos living on the surface of the earth before 1940 were mortal.” This seems fairly indubitable.
Bertrand Russell (from An Inquiry into Meaning and Truth, 1950)
[Indubitable though Russell’s amended proposition may be, it would be a mistake to suppose that we have come to the end of all logically possible objections that could be raised by a sceptic determined to make a nuisance of himself. In 2006 a newspaper article entitled ‘Indonesia’s Lost World’ reported that an expedition to a pristine tropical forest in an extremely remote region of Papua had discovered dozens of exotic new species of frogs, butterflies, palms, and even a primitive egg-laying mammal. Could there not, might ask our sceptic, be another such region somewhere in the world where a species of immortal dodo had managed to avoid attention? Supposing that we could rule out that strictly logical possibility by searching every square inch of space-time, it would still be premature to claim victory over this philosophical pest. For just when we finally thought he was about to go down for the count, he could always object with: “How do we know that there isn’t some undetected flaw in reason itself, and that such a flaw renders all inferences about the mortality of dodos unreliable?” We hope this discussion serves as a warning to anyone who thinks he can defeat single-minded scepticism, however perverse, with nothing more than unaided logic.]
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