A chimp in Kibale National Park, probably not thinking about the future.James Akena / Reuters

You’re holding a surprise party for a friend. The door opens, the lights flick on, everyone leaps out... and your friend stands there silent and unmoved. Now, you’re the one who’s surprised. You assumed she had no idea, and based on that, you made a (wrong) prediction about how she would react. You were counting on her ignorance. This ability to understand that someone else might be missing certain information about the world comes so naturally to us that describing it feels mundane and trite.

And yet, according to two psychologists, it’s a skill that only humans have. “We think monkeys can’t do that,” says Alia Martin from Victoria University of Wellington.

This claim is the latest volley in a long debate about how our fellow primates understand each other. Of particular interest is the question: Do they have a “theory of mind”—an understanding that others have their own mental states, their own beliefs and desires, their own ways of viewing the world?

Yes they do, say Martin and Laurie Santos from Yale University. But it’s different to ours in one crucial respect. The duo argue that other primates “have no concept of information that’s untrue or different [from] what they know.” That means, one, that they can’t conceive of states of the world that are decoupled from their current reality. And so, they can't imagine other individuals thinking about the world in a different way. They can think about the minds of others, but only when those minds have the same contents as theirs.

Put it this way: If a chimp sees other chimps staring at an apple on a ledge, it understands that they’re aware of the apple and might reach across to eat it—a basic theory of mind. But it can’t imagine what would happen if the apple was on the floor, or if the apple was a banana, or if its peers mistook the apple for something else.

“We might be the only species that can think about things that aren’t facts we have about the world, about other possible worlds, about states in the past or future, about counterfactuals,” says Santos. “We can simulate a whole fictional world. And if you’re a species that can get outside your own head, you can apply that to other people.” A chimp won't wonder if it'll be hungry tomorrow. It only cares if it's hungry now. An orangutan isn't going to write a novel, because this is the only reality that it knows.

Martin and Santos don’t make these claims lightly. In the 1990s, many scientists claimed that other primates don’t have a theory of mind at all, but based that claim on overly convoluted experiments. In the 2000s, by testing animals on simpler tasks that better reflect the challenges they naturally face, researchers showed that chimps know what their peers know, and can appreciate their goals and intentions.

But more recent studies have highlighted the differences between their theory of mind and ours. Imagine that a mother leaves a cookie beneath a plate, and her mischievous son hides it under the table. When she returns, he’s giggling to himself. He expects her to search for the cookie in the wrong place. If she doesn’t, and looks under the table instead, he is surprised. He might do a double-take. Indeed, that’s exactly what human infants do in this situation, from a very early age.

But it’s not what other primates do. Chimps and monkeys all consistently fail at these “false belief tasks.” For example, in 2011, Santos’s team played out a version of the cookie scenario (but using a lemon) in front of rhesus macaques. If the experimenter knew where the lemon was but looked in the wrong place, the monkeys stared longer at the scene—a sign that they had witnessed something unexpected. But if the experimenter had the wrong info about the lemon’s location and yet looked in the right place, the monkeys were nonplussed. “Even adult monkeys lack these skills that 12-month-old infants have,” says Santos.

Based on this work, Santos originally said that while non-human primates don’t understand false beliefs, they can reason about another individual’s knowledge and ignorance. That’s a view shared by other scientists who have done similar studies. But Santos now thinks that she was wrong. She and Martin argue that our closest relatives have no concept of ignorance at all. They know stuff. They can reflect on what they know. They can track if others know the same things as them. But if others don’t share the same knowledge, they’re at a loss. “They just have no prediction about what’s going on,” says Santos.

This is an odd and deeply counterintuitive concept. To Santos, it became clearer after reading about a study with chimps, published last year by Katja Karg and colleagues. In that set-up, each chimp saw a box outside its enclosure, with some bananas. A human experimenter would either hand the bananas across or keep them for herself. The chimp, meanwhile, could control what the human saw by manipulating covers that either kept the bananas in view, or hid them. A smart chimp with a full theory of mind would open the covers (or leave them open) for co-operative humans, while closing them (or leaving them closed) for competitors.

Karg’s chimps did show the food to the cooperative humans, but they didn’t hide the food from competitors.

“They were good at opening things that were already closed. They’re good at showing; they want others to have the same facts,” says Santos. “But they don’t know to shut something because they can’t predict what it would be like for someone to not know the same facts as them. In a sense, their theory of mind is really easy. They import their own understanding to someone else. But as soon as that information changes for someone else, they have no representation. They can’t simulate a world that’s not the world they’re in right now.”

Josep Call, who was involved in Karg’s study, likes the idea. Still, he notes that there are studies in which chimps will hide themselves from competitors, or even steal food after first moving their arms out of sight. But Santos argues that they can do all this without understanding their rival’s ignorance. All they know is whether the competitor has the same knowledge as them or not. If not, they don't know what the competitor is thinking, and so all bets are off. “The crazy thing is that this ability gets them really far,” she says. “They can deceive each other. This dumb system lets you be an organism that’s pretty good at Machiavellian tasks.”

“I feel it is too early for such a proposal,” says Frans de Waal from Emory University. Just because primates fail at current tasks doesn’t mean they’ll always do so—history has taught us that much. “Humans are usually tested by their own species, and the apes are tested by us, another species. This situation hampers them from the outset. Absence of evidence is not evidence of absence.”

“He’s right in that tomorrow someone could come up with the right study and show we’re all wrong, but so far things have been pretty consistent,” says Santos. Sarah Brosnan from Georgia State University agrees. “It fits the data, seemingly better than other current theories,” she says, “although finding empirical evidence to support this interpretation over the others will be tricky,  because of the challenges of testing these sorts of nuances.”

Martin and Santos already have ideas of possible experiments. Imagine that you show several containers to a chimp and then hide food in one of them. Now, another person comes in and goes straight to the hidden food. “That would be surprising to us—getting it right when you’re ignorant,” says Martin. “And the more containers there were, the more surprising it would be. But a monkey wouldn’t be surprised. Importantly, our theory can be falsified.”

This is all part of a broader philosophical debate about how we view other animals. Many have tried to show certain traits are uniquely human, perhaps in a narcissistic quest to establish our superiority over the rest of nature. Others, like de Waal, see more similarities than differences. “There is really no good evidence that we go about using tools, planning, or remembering in fundamentally different ways than other animals,” he says. “We often go beyond what they do, but the core processes are the same. My guess is that the same applies to theory of mind.”

Santos says she gets the similarities too. When watching other primates in the wild, “I think they’re going through the same things we are,” she says. “But I’m also struck by the fact that they’re doing it so differently. They’re not talking like we are. They’re not using technology like us. If you’re just looking to prop humans up and make them different, you’ll miss interesting things. But it’s the combination of the similarities and difference that will tell us the most.”

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.