A trolley is barreling toward five people tied to the tracks ahead. You can switch the trolly to another track—where only one person is tied down. What do you do? Or, more to the point, what does a self-driving car do?

Even the people building the cars aren’t sure. In fact, this conundrum is far more complex than even the pundits realize.

Now, more than ever, machines can learn on their own. They’ve learned to recognize faces in photos and the words people speak. They’ve learned to choose links for Google’s search engine. They’ve learned to play games that even artificial intelligence researchers thought they couldn’t crack. In some cases, as these machines learn, they’re exceeding the talents of humans. And now, they’re learning to drive.