 As is apparent from both his writings and his lectures, Molyneux was highly interested in optics and in the psychology of sight. To some extent this simply reflects the general attitude of his time: optics was a subject that was then engaging the attention of a number of leading scientists. His interest also had a more personal background, however, for his wife had lost her sight in the first year of their marriage. The immediate cause of his formulating the problem and sending it to Locke is to be found in Locke’s French extract of An Essay Concerning Humane Understanding, published in 1688 in the Bibliothèque Universelle &amp; Historique. In this extract Locke distinguished between ideas we acquire by means of one sense and those we acquire by means of more than one sense. He maintained that someone who lacks a sense will never be able to acquire the ideas pertaining to it. A blind man, for example, will never be able to have any idea of colour. Among the ideas we are able to acquire by means of a combination of senses, Locke reckoned those of space, rest, motion and figure. Molyneux’s problem had to do with the last of these. Molyneux was probably inspired by Locke’s exposition of the ideas of persons born blind and the ideas which can be acquired by means of both sight and touch. He was, moreover, a great admirer of Locke. (Although earlier we suggested that Molyneux might have been inspired by Ibn Tufail’s twelfth-century philosophical novel Hayy ibn Yaqdhan, which had just been published in Latin (1671) and English (1674) and also had an influence on Locke, there seems to be no indication that Molyneux read Ibn  Tufail.)[1]