Continuing in this slew of postings, here’s the third of my five essays in this week’s New York Times Magazine “Year in Ideas” issue:

Proving You’re Human

Want a free e-mail account at Yahoo? It’s yours, once you pass a little test. When you sign up, Yahoo now presents you with a small picture of a distorted word, something that looks as if it had been written on cellophane and stretched out of shape. If you can read the word and type it correctly into a response box, then you’ll get your free e-mail.

You will also have done something considerably weirder: you will have proved that you’re a human being.

The Yahoo test is intended to screen out ”spambots,” pieces of software that sign up for Yahoo e-mail addresses and then use them to send millions of pieces of junk mail. The test relies on a fundamental distinction between humans and machines: we can identify pictures, but robots can’t. ”Computers still can’t see things very well, but even very young kids are good at it,” says Manuel Blum, a Carnegie Mellon computer scientist who invented the test (though Hewlett-Packard holds the patent). ”They can read stuff on cereal boxes and bottles, curved words, upside down. No computer can do that.” Spam has become so ubiquitous that Blum’s technique has been eagerly adopted by e-commerce companies worldwide, including eBay and Ticketmaster. Other companies, like Spam Arrest and Knowspam, have introduced services that let you implement the test personally: the people who send you e-mail must first prove they’re human. If they can’t do that, they’re probably robots.

In 1950, the British mathematician Alan Turing imagined a ”Turing test” — in which a human tries to guess whether the typed messages he is receiving are from a person or a computer. In Turing’s day, this was merely an effete philosophical question, but these days it’s a basic task of everyday life, equal parts humdrum and surreal. Is that a message from a spambot? Or maybe your sister? In a world crawling with so many forms of pseudo-life, you need a machine to help you figure out who’s human. — Clive Thompson