What does the word ‘recursion’ mean?

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Increase your vocabulary and you’ll make your writing much more precise. That’s why I provide a word of the week. Today’s word: recursion…

What happens when medical diagnoses become automated? That was the question that physician and writer Siddhartha Mukherjee (author of the superb book The Emperor of all Maladies) addressed in an April 2017  New Yorker piece headlined, “A.I. vs M.D.

I’m a longtime fan of the New Yorker and was one of my favourite pieces of 2017; I wrote about it earlier, lauding its superb use of figurative language. But in the same piece, Mukherjee also gave me my word of the week, recursion. Here is how he used it:

Twenty-five such radiologist were asked to evaluate X-rays of the lung while inside MRI machines that could track the activities of their brains. (There’s a marvellous series of recursions here: to diagnose diagnosis, the imagers had to be imaged.)

Recursion occurs when a thing is defined in terms of itself or of its type. The most common application of recursion is in mathematics and computer science, where a function being defined is applied within its own definition. More simplistically, you might see a recursion as repetition. This is illustrated in the photograph above where you can see the image of someone wearing sunglasses multiplied numerous times in the image of the sunglasses themselves. (This act of repetition is also known as the Droste effect.)

The etymology of recursion is utterly straightforward. It is from the Latin recursionem meaning  “a running backward, return.” The root is the same for the better-known verb “to recur.”

An earlier version of this post first appeared on my blog on July 12/17.

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