When Descartes tells us that sensation reduced to itself is always true, and that error creeps in through the transcendent interpretation of it that judgement provides, he makes an unreal distinction: it is no less difficult for me to know whether or not I have felt something than it is to know whether there is really something there, for the victim of hysteria feels yet does not know what it is that he feels, as he perceives external objects without being aware of that perception.