Sometimes known as the use of hyperbolic (extreme) doubt, or Cartesian doubt. This is the method of investigating the extent of knowledge and its basis in reason or experience used by Descartes in the first two Meditations. It attempts to put knowledge upon a secure foundation by first inviting us to suspend judgement on any proposition whose truth can be doubted, even as a bare possibility. The standards of acceptance are gradually raised as we are asked to doubt the deliverance of memory, the senses, and even reason, all of which are in principle capable of letting us down. The process is eventually dramatized in the figure of the evil demon, or malin génie, whose aim is to deceive us, so that our senses, memories, and reasonings lead us astray. The task then becomes one of finding a demon-proof point of certainty, and Descartes produces this in the famous ‘Cogito ergo sum’: I think therefore I am. It is on this slender basis that the correct use of our faculties has to be reestablished, but it seems as though Descartes has denied himself any materials to use in reconstructing the edifice of knowledge. He has a basis, but no way of building on it without invoking principles that will not be demon-proof, and so will not meet the standards he has apparently set himself. It is possible to interpret him as using ‘clear and distinct ideas’ to prove the existence of God, whose benevolence then justifies our use of clear and distinct ideas (‘God is no deceiver’): this is the notorious Cartesian circle. Descartes’s own attitude to this problem is not quite clear: at times he seems more concerned with providing a stable body of knowledge that our natural faculties will endorse, rather than one that meets the more severe standards with which he starts out. For example, in the second set of Replies he shrugs off the possibility of ‘absolute falsity’ of our natural system of belief, in favour of our right to retain ‘any conviction so firm that it is quite incapable of being destroyed’. The need to add such natural belief to anything certified by reason is eventually the cornerstone of Hume’s philosophy, and the basis of most 20th-century reactions to the method of doubt. See also naturalized epistemology.