That madness be posited as disqualifying in any search for the truth, that it should not be 'reasonable' to call it to oneself to carry out the necessary doubting, that one cannot feign it even for an instant, and that the impossibility appears immediately in the very assignation of the term 'demens'; this is indeed the decisive point where Descartes departs from all those for whom madness may be in some way or other a bearer or a revealer of truth.