In 1962, Thomas Kuhn published The Structure of Scientific Revolutions and caused a small revolution of his own. He profoundly changed the way we now understand scientific knowledge, invoking the notion of a paradigm, and then claiming that we can only make sense of the world around us through the paradigm’s lens. A paradigm, for Kuhn, is the product of consensus. It’s the whole complex set of things that everyone (or at least every scientist) agrees about at any given time. When we change paradigms, our whole world view changes.

Groping for a way to illustrate this, Kuhn turned to a familiar optical illusion. Look at a picture one way and you see a rabbit. Look another way and you see a duck. Moving from one scientific world view to another, he said, was just like this. Once you had convinced yourself that what you saw was a duck, a duck was all you could see; making yourself see that rabbit again could be very difficult.

Using an optical illusion to explain how knowledge works might seem like a peculiar thing to do, but Kuhn was not the first to deploy the tactic in making an epistemological argument. In Philosophical Investigations (1953), Ludwig Wittgenstein used the same illusion to illustrate the difference between ‘seeing’ and ‘seeing as’— or the difference between seeing an object, and recognising it as some specific thing; many philosophers argue that you can’t be said to see something without seeing it as something. In turn, Wittgenstein got it from the Polish-American psychologist Joseph Jastrow’s Fact and Fable in Psychology (1900).

The connection between seeing and knowing has a long history, of course. It is no coincidence that we often say ‘I see’ when what we mean is ‘I understand’. But using optical illusions as a way of talking about knowledge, its limits, and its relationship to authority has a history, too. The Victorians, in particular, were fascinated by optical illusions and performances. Though we now live in a world that is saturated with illusions, many of them have their origins in that Victorian world. And so looking at how the Victorians played with vision can give us a handle on what contemporary illusions can tell us about the ways we see knowledge.

When the Scottish natural philosopher David Brewster published his Letters on Natural Magic in 1832, he was placing himself in a long tradition of visual deception. Brewster was well-known to early 19th-century readers as an authority on optics, an opponent of the wave theory of light and a stern critic of the old regime at the Royal Society. He was also the inventor of the kaleidoscope. Brewster expounded a Scottish common-sense philosophy that trumpeted the reliability of the senses as conduits of knowledge, so evading Humean scepticism on the one hand, and Kantian idealism on the other. His book on natural magic, written as a riposte to his friend Sir Walter Scott’s Letters on Demonology and Witchcraft (1830), set out to demystify ghosts and apparitions by putting them in their place in the natural world. To do that, he had to lay bare the mechanism of illusion.

Brewster knew very well that deceiving the senses was traditionally a way of fooling people, erecting smoke and mirrors to obfuscate something unpalatable, such as jumped-up ideology. ‘When the tyrants of antiquity were unable or unwilling to found their sovereignty on the affections and interests of their people,’ he wrote, then the ‘prince, the priest, and the sage were leagued in a dark conspiracy to deceive and enslave their species.’ Exposing just how duplicitous priests filled their temples with visions was thus a way of maintaining proper political order.

But playing with illusions also offered a way of experimenting with knowledge and its limitations. Brewster was in no doubt that seeing was knowing, but he was just as sure that seeing properly was also an exercise in judgment. Understanding how deception worked offered a way of educating the eye and mind to see through illusion. This made optics an anti-authoritarian science — but also one that depended on knowing whom to trust with knowledge.

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  1. Illusions can explain how knowledge works.


  2. MI1: we can make sense of the world through a paradigm
    MI2: relation b/w illusions and knowledge = historical


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