"De omnibus dubitandum, taught Descartes - doubt all things. Easily said - but how to do it? Try, for example to doubt that the laws of nature are always binding: one day a case may occur where nature makes an exception for some stone, and exempts it from the law of gravity. But how to find this stone, if one has the courage to admit such a possibility, even if one knew definitely that such a stone existed? p. 215"