Such so-called "dark" ads are usually sent to the very people most likely to be susceptible to them. Unlike old-school pamphleteering and letterboxing, the ads aren't distributed helter-skelter. They're targeted, based on in-depth mining of people's browsing histories, Facebook likes, tweets, and online purchases. What's more, a dark ad is typically sent without the receiver having the benefit of hearing the opposing view.

This isn't how the democratic "marketplace of ideas" is supposed to work. Indeed, how we're to understand and regulate the influence of algorithms on our perceptions is among the most important questions AI poses today