Molyneux’s Problem
Core Question
Molyneux’s problem is a classic question in philosophy of perception: Would a person, born blind and given sight as an adult, be able to visually distinguish a sphere from a cube that they previously knew only by touch?
The problem challenges whether spatial concepts are amodal (shared across senses) or modality-specific (learned separately for each sense). It has generated over 300 years of philosophical debate and empirical investigation.
Original Formulations
Molyneux’s Letter to Locke (7 July 1688)
“A Problem Proposed to the Author of the Essai Philosophique concernant L’Entendement: A Man, being born blind, and having a Globe and a Cube, nigh of the same bignes, Committed into his Hands, and being taught or Told, which is Called the Globe, and which the Cube, so as easily to distinguish them by his Touch or Feeling; Then both being taken from Him, and Laid on a Table, Let us Suppose his Sight Restored to Him; Whether he Could, by his Sight, and before he touch them, know which is the Globe and which the Cube? Or Whether he Could know by his Sight, before he stretch’d out his Hand, whether he Could not Reach them, tho they were Removed 20 or 1000 feet from Him?”
— William Molyneux (1656–1698), Irish scientist and politician
Locke’s Published Version (1694, Essay 2nd Edition)
Locke was initially silent on Molyneux’s letter (1688), but after a second letter (March 1693) enthusiastically included it as a supporting argument in the second edition of his Essay Concerning Human Understanding (1694). Locke’s version simplified the question slightly, asking only whether the newly-sighted person could “distinguish, and tell, which is the Globe, which the Cube.”
Historical Context
William Molyneux was motivated in part by his wife’s blindness, which began during their first year of marriage. The problem was inspired by Locke’s discussion in the Essay of whether ideas can enter the mind through a single sense or require multiple senses.
A possible precursor is Ibn Tufail’s 12th-century philosophical novel Hayy ibn Yaqdhan (translated into Latin in 1671 and English in 1674), which describes a spontaneously-generated human on a desert island who acquires knowledge through sensory experience. However, there is no evidence Molyneux read this work.
18th-Century Philosophical Positions
| Philosopher | Answer | Rationale |
|---|---|---|
| Molyneux, Locke, Berkeley | No (negative) | Empiricist: the visual-tactual relation is arbitrary, learned only through experience |
| **Synge, Lee, [[gottfried-wilhelm-leibniz | Leibniz]]** | Yes (affirmative) |
| Reid | Mixed | Visual and tactual notions differ but can be related by reason |
| Boullier, Hutcheson | Affirmative | Visual and tactual notions share something observed directly |
The disagreement among 18th-century philosophers stemmed partly from different interpretations of the problem — whether the subject could use memory or reason, walk around the objects, or be told in advance what would appear.
First Experimental Data: Cheselden (1728)
Surgeon William Cheselden (1688–1752) operated on Daniel Dolins, a congenitally blind patient with cataracts. The boy could not distinguish shapes by sight initially and could not recognise objects regardless of shape or magnitude differences.
Interpretations varied widely:
- Voltaire, Camper, Condillac — confirmed Berkeley’s theory: the blind must learn to see
- La Mettrie, Diderot — ambiguous; noted confounding factors: eyes not functioning properly post-surgery, insufficient recovery time, leading questions
- Mérian — criticised that cataracts don’t cause complete blindness, but patients operated for cataracts are still relevant as they cannot perceive form before operation
19th-Century Developments
The debate fragmented into four distinct approaches:
- Cataract patient studies — conflicting results with widely varying pre/post-operative conditions
- Animal studies (Adam Smith, Johannes Müller) — some animals see at birth, supporting the affirmative answer
- Infant studies (Thomas Brown) — babies must learn to see, supporting the negative answer
- Wheatstone’s stereoscope — discovery of immediate 3D space perception, supporting the affirmative and challenging Berkeley
The key shift was from asking about whether cross-modal recognition is possible to whether space perception is innate or acquired.
Modern Approaches
Clinical Studies
Accounts of recovery from early blindness remain problematic and inconclusive — the number of cases is small, conditions vary enormously, and modern ethics prevent controlled deprivation studies.
Visual Deprivation in Animals
Studies reveal a critical period for visual system development. A key limitation: Molyneux’s original thought experiment assumed a good visual system (just untrained), whereas deprived animals have abnormal visual systems. Used as evidence for Locke’s empiricist position.
Sensory Substitution Devices
Devices that present visual information via touch or audition using novel coding schemes show that subjects need time to learn — supporting the Molyneux/Locke negative answer. However, the coding schemes are arbitrary, so the interpretation is contested.
Crossmodal Correspondences
Modern research on crossmodal correspondences — including the Bouba/Kiki effect — challenges the strict empiricist view by demonstrating non-arbitrary mappings between sensory modalities. If shape-sound mappings are universal and likely innate, shape-touch-to-shape-vision mappings might also be partially available without learning.
Glenney’s “New Molyneux Problem” (2024)
Brian Glenney argues that the traditional Molyneux question is:
- Untestable — a genuine Molyneux subject (congenitally blind from birth, then fully sighted) cannot be physically realised
- Underspecified — it uses a simplistic taxonomy of “the blind” that groups ocular, cortical, and psychological blindness into a singular ontology
- Level-dependent — the answer depends on the domain of inquiry
Proposed solution: Explanatory pluralism. The answer is simultaneously no, yes, and “no answer” depending on the explanatory level (intentional, physical, or design stance, following Dennett). Glenney draws on previously unheeded insights from animal crossmodal perception studies — dolphins, vervet monkeys, bees, chicks — and from the Bouba/Kiki effect via Uznadze’s finding that 95% of participants match “bouba” to round shapes and “kiki” to sharp shapes, demonstrating universal crossmodal correspondences.
Relevance to Neuroscience and Vision Science
Molyneux’s problem intersects with several topics in the wiki:
- The Bouba/Kiki effect — crossmodal correspondences as evidence against strict empiricism
- Prediction-based accounts of perception — if perception is fundamentally predictive (the brain generates sensory predictions from multimodal models), then cross-modal transfer may be partially available at birth
- Active inference — the brain actively constructs coherent interpretations of sensory input across modalities, suggesting some supramodal spatial representations might be innate
References
- Degenaar, M. & Lokhorst, G.J. (2024). Molyneux’s Problem. In E.N. Zalta & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 Edition). 1
- Glenney, B. (2024). The New Molyneux Problem. In A Pluralist’s Guide to Solving Molyneux’s Problem, pp. 118-138. Routledge. doi:10.4324/9781003255512-6. 2