But, while my senses may deceive me about what is small or far away, there may still be other things taken in by the senses which I cannot possibly doubt ... unless perhaps I liken myself to madmen whose brains are so rattled by the persistent vapors of melancholy that they are sure they are kings when in fact they are paupers, or that they wear purple robes when in fact they are naked, or that their heads are clay, or that they are gourds, or that they are made of glass. But these people are insane, and I would seem just as crazy if I were to apply what I say about them to myself.

René Descartes, Meditations on First Philosophy, Meditation 1

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