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Stop and think for a moment, if you would, about the way you think. Maybe before leaving home this morning, you thought to yourself, Don’t forget to turn off the oven! Or, perhaps, after leaving, you said to yourself, “Damn, I forgot my keys.” It feels natural enough, but in his new book, The Voices Within, Durham University psychologist Charles Fernyhough asks us to consider something most of us take for granted: Why do we think in words at all?

It’s an intriguing question, and it’s one that Fernyhough — the director of Hearing the Voice, a multidisciplinary study on inner voices — is more than qualified to answer. He notes that roughly 20 to 25 percent of waking time is spent with thoughts like these, language directed at ourselves. Some people keep up a patter with themselves for even more time. But couldn’t we just imagine those keys, or that knob on the oven being turned down? After all, the easy answer for why language exists is that it fosters communication between people. If a husband tells his wife “You look sick, maybe you should stay home,” words are helpful because those ideas are difficult to get across without them. But if his wife thinks to herself I called out last week, my manager might be mad if I call out again, language doesn’t seem like it should be necessary — she’s the only one present. Rats in experiments solve mazes without the words “Remember that left turn.” So why do human brains so often rely on language in order to reason and remember?

Fernyhough has an answer in mind, but he first runs down several functions of talking to oneself. The first is motivation and focusing. Among athletes, self-talk is common. A meta-analysis found improved performance for athletes who used self-talk, especially in sports with fine motor technique like golfing and gymnastics. (Look at Laurie Hernandez at the Olympics for less scientific but more entertaining evidence.) Certainly to a best man, encouraging himself in the mirror before his speech in an attempt to focus feels helpful. Words can, it seems, help regulate our mood and direct our attention. Another purpose of self-talk could be changing specific behavior, like telling yourself “Stop fidgeting” during a job interview, or “Remember to follow through on your serve!” Through language, we regulate what our bodies do in the world, as if our minds are privately conversing with our bodies.

Still, it’s not yet clear why these thoughts need to be in words at all. Isn’t it possible to motivate yourself or improve your form by simply visualizing yourself acing that serve, rather than telling yourself you will?

To Fernyhough, language is a special tool because it can be “dialogic” — that is, between different perspectives. Humans begin speaking as children in dialogue with their caregivers, and because, fundamentally, language begins as a conversation between two people, it retains some of the special traits of those dialogues even when it’s in our own head. “When you talk to yourself,” as Fernyhough phrases it, “you step out of yourself for a moment and get some perspective on what you are doing.”

An easy-to-grasp example is what you could call your conscience. What’s a mental conversation about whether to eat that third slice of pizza besides different perspectives battling within one’s own mind? This is Fernyhough’s main insight: It’s easy to imagine certain thoughts without language, like visualizing that stove being left on, but it’s difficult to imagine tension or debate without language, and a great deal of human thought is deciding between options. As he writes, “Language is particularly powerful at representing different perspectives and bringing them into contact with each other.”

This sounds plausible enough, but there’s also some some fascinating neurological evidence to support it. In one experiment conducted at Hearing the Voice, participants were given a scenario, like returning to their alma mater, and then told to imagine having either a monologue (like delivering a speech) or dialogue (talking to an old professor). Though this study is only prompting imagined thoughts rather than observing them, neuroimaging showed that in the scenarios involving dialogues, areas activated that “are particularly associated with thinking about other minds, or so-called ‘theory of mind’ abilities.” In other words, dialogic thoughts might have special properties because the brain acts as if they are conversations. A friend you practiced interviewing with could have told you “stop fidgeting”; a coach might well tell you to follow through on that serve. But they don’t need to, because your brain is capable of mimicking those conversations for you. Perhaps this is what’s happening when you debate with yourself about whether you can put off leaving for the airport.

Fernyhough is careful to couch these claims as not yet proven. Studies like the above aren’t perfect, because it’s difficult to study thoughts without relying on self-reported introspection, not to mention memory. There’s no reason to believe asking a participant “What were you thinking when you caught that ball?” will provide an accurate answer. Another caveat is that “language” is a useful shorthand, but “communication system” might be more accurate; he notes a deaf respondent on Quora reporting that the language he or she saw when thinking was either “ASL [American Sign Language] signs, or pictures, or sometimes printed words.”

But after reading the book, I couldn’t help noticing my thoughts more closely — asking myself, Is this dialogic thinking? or What perspective was that voice taking? At one point, there’s mention of “the idea that, when we internalise dialogue, we internalise other people. Our brains, like our minds, are full of voices.” For me, at least for now, one of those voices is Fernyhough’s.