Molyneux writes to H. P. Grice

Molyneux’s problem Epistemology A problem about the correlation between sight and touch, proposed by the Irish politician and scientist William Molyneux (1656–98) in a letter addressed to Locke, and which is included by Locke in the second edition of Essay Concerning Human Understanding (ii, ix, 8). Suppose a blind person has learned to distinguish a cube from a sphere of the same metal by the sense of touch. If the person is suddenly made to see, can he immediately distinguish the two objects by sight before touching them? Both Molyneux and Locke answered this question in the negative. They believed that our ordinary perceptions depend on judgments based on experience. A perceiver must learn to build perceptual knowledge by correlating the contents from different channels. Berkeley agreed with this solution but claimed that it proved his own thesis that the data of touch and the data of sight are heterogeneous. Leibniz also discussed this problem, but derived a different answer. He suggested that the two sets of experience have one element in common, that is, extension. Hence it is possible to infer from one type of idea to another. Empirical testing seems to favor Locke’s solution. “A farther confirmation of our tenet may be drawn from the solution of Mr. Molyneux’s problem, published by Mr. Locke in his Essay . . . that the blind man at first sight would not be able with certainty to say which was the globe which the cube, whilst he only saw them.” Berkeley, An Essay Towards a New Theory of Vision