The Biggest Questions: Is it possible to really understand someone else's mind?

That's because machine-learning algorithms need both brain signals and information about what they correspond to, paired in perfect synchrony, to learn what the signals mean. When studying inner experience, all scientists have to go on is what people say is going on inside their head, and that can be reliable. "It's not like it's directly measuring as a ground truth what people experienced," says Raphaël Millière, a lecturer in philosophy at Macquarie University in Australia. Tying brain activity to subjective experience requires facing up to the slipperiness and inexactitude of language, particularly when deployed to capture the richness of one's inner life. In order to meet that demanding brief, scientists like Millière are marrying contemporary artificial intelligence with centuries-old techniques, from philosophical interview strategies to ancient meditation practices.