Locke's Irish friend William Molyneux posed this problem...

Locke's Irish friend William Molyneux posed this problem with precision in letters to the Bibliothèque Universelle and to Locke himself. [Correspondence 1064, 1609, 1620.] Supposing that someone blind from birth became familiar with solid figures by touch alone and then later gained the power to see, would this person be able to distinguish a cube and a sphere by sight alone without first touching them?

In a passage added to the Essay's second edition, Locke agreed with Molyneux (on a view confirmed by twentieth-century empirical research) that the answer must be no. [Essay II ix 8] The visual and tactile ideas of the globe are distinct. Although we use…