Tesla chief executive Elon Musk has warned about artificial intelligence before, tweeting that it could be more dangerous than nuclear weapons. Speaking Friday at the MIT Aeronautics and Astronautics department’s Centennial Symposium, Musk called it our biggest existential threat:

I think we should be very careful about artificial intelligence. If I were to guess like what our biggest existential threat is, it’s probably that. So we need to be very careful with the artificial intelligence. Increasingly scientists think there should be some regulatory oversight maybe at the national and international level, just to make sure that we don’t do something very foolish. With artificial intelligence we are summoning the demon. In all those stories where there’s the guy with the pentagram and the holy water, it’s like yeah he’s sure he can control the demon. Didn’t work out.

Musk was so caught up on artificial intelligence that he missed the audience’s next question. “Sorry can you repeat the question, I was just sort of thinking about the AI thing for a second,” he said.

Musk spoke expansively for over an hour, at one point even asking a MIT student what his favorite sci-fi books were. He left to a standing ovation. You can watch the entire interview here.