Two paragraphs from Ludwig Wittgenstein's On Certainty, which he wrote in the last two years of his life. A translation is also available online in its entirety.

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185. It would strike me as ridiculous to want to doubt the existence of Napoleon; but if someone doubted the existence of the earth 150 years ago, perhaps I should be more willing to listen, for now he [sic] is doubting our whole system of evidence. It does not strike me as if this system were more certain than a certainty within it.

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676. "But even if in such cases I can't be mistaken, isn't it possible that I am drugged?" If I am and if the drug has taken away my consciousness, then I am not now really talking and thinking. I cannot seriously suppose that I am at this moment dreaming. Someone who, dreaming, says "I am dreaming", even if he speaks audibly in doing so, is no more right than if he said in his dream "it is raining", while it was in fact raining. Even if his dream were actually connected with the noise of the rain.