Optical connections: William Molyneux’s thought experiment

William Molyneux had a problem. Or at least his scientific and philosophical investigations led him to propose the conundrum that now bears his name. Molyneux’s thought-experiment (as it could only have been in his time) addresses the very nature of visual perception. It has engaged many of the finest thinkers from the 17th century onwards, notably John Locke and Bishop George Berkeley, but also Leibnitz, Voltaire and Diderot to name just a few.

The background to Molyneux’s eponymous query was both his wide scientific studies and an episode of personal tragedy. He was born in Dublin in 1656 to a wealthy English Protestant landowner. An aptitude for science and mathematics was amply demonstrated by the time he graduated from the city’s Trinity College aged 17. One of his interests was optics; he struck up a friendship with the Astronomer Royal, John Flamsteed, to whom he complained that good optical instruments were hard to come by in Ireland. And one of Molyneux’s best-known works is his optical treatise, published in 1692, Dioptrica Nova. The previous year his wife had died, after only 13 years of marriage. Lucy, ‘a lady noted for intelligence, amiability and great beauty’ according to one biographer of Molyneux, was the daughter of the Attorney-General of Ireland, Sir William Domville. Within two months of their wedding Lucy was taken ill and lost her sight, the doctors being able to find neither cause nor cure. Despite suffering in pain throughout the rest of her short life, the couple had three children, although only one survived childhood.