ABSTRACT

It is a remarkable, perhaps astonishing, fact that nothing comes nearer in the Essay to being a discussion of sensory illusion than the famous account of 'how the same Water, at the same time, may produce the Idea of Cold by one Hand, and of Heat by the other: Whereas it is impossible, that the same Water, if those Ideas were really in it, should at the same time be both Hot and Cold'. Locke's explanation of this phenomenon is strongly reminiscent of Gassendi's in his argument that impressions are always true. It is that ideas of heat or cold correspond, not to the absolute degree of motion of the particles either in objects or in our organs of touch, but to 'the increase or diminution' of motion in the latter 'caused by the Corpuscles of any other Body'." Since a simple idea represents (is a sign of) its normal cause, Locke was not so much explaining a contradiction between different deliverances of the senses as explaining it away: he represented both sensations as in effect veridical. The existence of this Epicurean line of thought may help to explain the neglect of illusion in the discussion of sensitive knowledge, but another factor, perhaps, was Descartes' having moved the centre of

the dispute with the sceptic from the ancient topics of sticks that look bent and round towers that look square to the possibility of a coherent and interminable dream or deception in the presence of nothing at all.