What would an adult who had been blind all his life be able to see, if the cause of his blindness was suddenly removed? This question was asked by several empiricist philosophers. John Locke considered the possibilities of such a case in 1690, following the question posed in a letter from his friend William Molyneux:
Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere of the same metal. Suppose then the cube and the sphere were placed on a table, and the blind man made to see: query, whether by his sight, before he touched them, could he distinguish and tell which was the globe and which the cube? . . . The acute and judicious proposer answers: not. For though he has obtained the experience of how the globe, how the cube, affects his touch, yet he has not yet attained the experience that what affects his touch so or so, must affect his sight so or so . . .