Every philosopher had an opinion. It took 315 years and five surgeries to get the actual answer. Nobody was ready for it.

In 1688, an Irish scientist named William Molyneux wrote a letter to the philosopher John Locke.

Inside was a riddle.

It sounded simple.

It was not.

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Sources

1. Molyneux, W. (1688). Letter to John Locke. Reproduced in Locke, J. "An Essay Concerning Human Understanding," Book II, Ch. IX.
2. 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.
3. Sinha, P. (2013). "Once Blind and Now They See." Scientific American, 309(1), 48–55.
4. Project Prakash, MIT Department of Brain and Cognitive Sciences. web.mit.edu/bcs/sinha.
5. Locke, J. (1689). "An Essay Concerning Human Understanding." Book II, Ch. IX, Sec. 8.
6. Leibniz, G.W. (1704). "New Essays on Human Understanding." Response to Molyneux's Problem.
7. Sacks, O. (1985). The Man Who Mistook His Wife for a Hat and Other Clinical Tales. Summit Books / subsequent editions.