Molyneux's problem is a thought experiment in philosophy concerning immediate recovery from blindness. It was first formulated by William Molyneux, and notably referenced in John Locke's An Essay Concerning Human Understanding. The problem can be stated in brief, "if a man born blind can feel the differences between shapes such as spheres and cubes, could he, if given the ability to see (but now without recourse to touch), distinguish those objects by sight (alone), in reference to the tactile schemata he already possessed?"
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, and nighly of the same bigness, so as to tell, when he felt one and the other, which is the cube, which is the sphere. Suppose then the cube and the sphere placed on a table, and the blind man made to see: query, Whether by his sight, before he touched them, he could now distinguish and tell which is the globe, which the cube? To which the acute and judicious proposer answers: 'Not. For though he has obtained the experience of how a globe, and how a 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…'
I agree with this thinking gentleman, whom I am proud to call my friend, in his answer to this problem; and am of opinion 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; though he could unerringly name them by his touch, and certainly distinguish them by the difference of their figures felt.
In 1709, in A New Theory of Vision, George Berkeley also concluded that there was no necessary connection between a tactile world and a sight world—that a connection between them could be established only on the basis of experience. He speculated:
the objects to which he had hitherto used to apply the terms up and down, high and low, were such as only affected or were in some way perceived by touch; but the proper objects of vision make a new set of ideas, perfectly distinct and different from the former, and which can in no sort make themselves perceived by touch— (sect. 95).
In 1749, Denis Diderot wrote Letter on the blind for the benefit of those who see as a criticism of our knowledge of ultimate reality.
Locke himself believed that sight and touch were entirely different sense perceptions and thus the blind man could not distinguish the sphere from the cube at first sight.
A similar problem was also addressed earlier in the 12th century by Ibn Tufail (Abubacer), in his philosophical novel, Hayy ibn Yaqdhan (Philosophus Autodidactus). His version of the problem, however, dealt mainly with colors rather than shapes.
Production of speech is seen as a pure motor act, involving muscles and the neurons controlling them, while perception of speech is seen as purely sensory, involving the ear and the auditory pathway. This parcellation of the systems appear intuitive and clear, but recent studies [beginning with Taine 1870!] … suggest that such divisions may be fundamentally wrong. Rather than separate processes for motor outputs and individual sensory modalities, adaptive action seems to use all the available context-specific information. That is, neural representations across the brain may be centered on specific actions. This view on neural representations puts 'Molyneux's Problem' in a new light. Unisensory signals are fused into multisensory motor representations unified by an action, but since Molyneux does not suggest any action, his 'problem' may be better viewed as an ill-posed question—at least from a neuroscientific perspective.— 
One reason that Molyneux's Problem could be posed in the first place is the extreme dearth of human subjects who gain vision after extended congenital blindness. Alberto Valvo estimated that fewer than twenty cases have been known in the last 1000 years. Ostrovsky, et al., studied a woman who gained sight at the age of 12 when she underwent surgery for dense bilateral congenital cataracts. They report that the subject could recognize family members by sight six months after surgery, but took up to a year to recognize most household objects purely by sight.
In 2003, Pawan Sinha, a professor at the Massachusetts Institute of Technology, set up a program in the framework of the Project Prakash and eventually had the opportunity to find five individuals who satisfied the requirements for an experiment aimed at answering Molyneux's question experimentally. Prior to treatment, the subjects (aged 8 to 17) were only able to discriminate between light and dark, with two of them also being able to determine the direction of a bright light. The surgical treatments took place between 2007 and 2010, and quickly brought the relevant subject from total congenital blindness to fully seeing. A carefully designed test was submitted to each subject within the next 48 hours. Based on its result, the experimenters concluded that the answer, in short, to Molyneux's problem is "no". Although after restoration of sight, the subjects could distinguish between objects visually almost as effectively as they would do by touch alone, they were unable to form the connection between an object perceived using the two different senses. The correlation was barely better than if the subjects had guessed. They had no innate ability to transfer their tactile shape knowledge to the visual domain. However, the experimenters could test three of the five subjects on later dates (5 days, 7 days, and 5 months after, respectively) and found that the performance in the touch-to-vision case improved significantly, reaching 80–90%.
- John Locke, An Essay Concerning Human Understanding, book 2, chapter 9
I shall here insert a problem of that very ingenious and studious promoter of real knowledge, the learned and worthy Mr. Molyneux, which he was pleased to send me in a letter some months since; and it is this:—"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, and nighly of the same bigness, so as to tell, when he felt one and the other, which is the cube, which the sphere. Suppose then the cube and sphere placed on a table, and the blind man be made to see: quaere, whether by his sight, before he touched them, he could now distinguish and tell which is the globe, which the cube?" To which the acute and judicious proposer answers, "Not. For, though he has obtained the experience of how a globe, how a cube affects his touch, yet he has not yet obtained the experience, that what affects his touch so or so, must affect his sight so or so; or that a protuberant angle in the cube, that pressed his hand unequally, shall appear to his eye as it does in the cube."—I agree with this thinking gentleman, whom I am proud to call my friend, in his answer to this problem; and am of opinion 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; though he could unerringly name them by his touch, and certainly distinguish them by the difference of their figures felt. This I have set down, and leave with my reader, as an occasion for him to consider how much he may be beholden to experience, improvement, and acquired notions, where he thinks he had not the least use of, or help from them. And the rather, because this observing gentleman further adds, that "having, upon the occasion of my book, proposed this to divers very ingenious men, he hardly ever met with one that at first gave the answer to it which he thinks true, till by hearing his reasons they were convinced.
- "TO SEE AND NOT SEE". Archived from the original on 2006-08-31. Retrieved 2010-05-04.
- Muhammad ibn Abd al-Malik Ibn Tufayl and Léon Gauthier (1981), Risalat Hayy ibn Yaqzan, p. 5, Editions de la Méditerranée:
If you want a comparison that will make you clearly grasp the difference between the perception, such as it is understood by that sect [the Sufis] and the perception as others understand it, imagine a person born blind, endowed however with a happy natural temperament, with a lively and firm intelligence, a sure memory, a straight sprite, who grew up from the time he was an infant in a city where he never stopped learning, by means of the senses he did dispose of, to know the inhabitants individually, the numerous species of beings, living as well as non-living, there, the streets and sidestreets, the houses, the steps, in such a manner as to be able to cross the city without a guide, and to recognize immediately those he met; the colors alone would not be known to him except by the names they bore, and by certain definitions that designated them. Suppose that he had arrived at this point and suddenly, his eyes were opened, he recovered his view, and he crosses the entire city, making a tour of it. He would find no object different from the idea he had made of it; he would encounter nothing he didn’t recognize, he would find the colors conformable to the descriptions of them that had been given to him; and in this there would only be two new important things for him, one the consequence of the other: a clarity, a greater brightness, and a great voluptuousness.
- Diana Lobel (2006), A Sufi-Jewish Dialogue: Philosophy and Mysticism in Baḥya Ibn Paqūda's Duties of the Heart, p. 24, University of Pennsylvania Press, ISBN 0-8122-3953-9.
- Ghazanfar, A. A. & Turesson, H. K. (2008). Speech production: How does a word feel? Current Biology, 18,24: R1142-1144.
- Valvo, A. (1971). Sight restoration after long-term blindness: The problems and behavior patterns of visual rehabilitation. New York: American Foundation for the Blind.
- Ostrovsky, et al., "Vision following extended congenital blindness", Department of Brain and Cognitive Sciences Massachusetts Institute of Technology
- Held, R.; Ostrovsky, Y.; De Gelder, B.; Gandhi, T.; Ganesh, S.; Mathur, U.; Sinha, P. (2011). "The newly sighted fail to match seen with felt". Nature Neuroscience 14 (5): 551–553. doi:10.1038/nn.2795. PMID 21478887.
- Scientists settle centuries-old debate on perception
- Mapping touch to sight takes time to learn
- Taine, Hippolyte (1870). De l'intelligence. Paris.