“What is the difference between yesterday and tomorrow?”

The Yupno man we were interviewing, Danda, paused to consider his answer. A group of us sat on a hillside in the Yupno Valley, a remote nook high in the mountains of Papua New Guinea. Only days earlier we had arrived on a single-engine plane. After a steep hike from the grass airstrip, we found ourselves in the village of Gua, one of about 20 Yupno villages dotting the rugged terrain. We came all the way here because we are interested in time—in how Yupno people understand concepts such as past, present and future. Are these ideas universal, or are they products of our language, our culture and our environment?

As we interviewed Danda and others in the village, we listened to what they said about time, but we paid even closer attention to what they did with their hands as they spoke. Gestures can be revealing. Ask English speakers about the difference between yesterday and tomorrow, and they might thrust a hand over the shoulder when referring to the past and then forward when referring to the future. Such unreflective movements reveal a fundamental way of thinking in which the past is at our backs, something that we “leave behind,” and the future is in front of us, something to “look forward” to. Would a Yupno speaker do the same?

Danda was making just the kinds of gestures we were hoping for. As he explained the Yupno word for “yesterday,” his hand swept backward; as he mentioned “tomorrow,” it leaped forward. We all sat looking up a steep slope toward a jagged ridge, but as the light faded, we changed the camera angle, spinning around so that we and Danda faced in the opposite direction, downhill. With our backs now to the ridge, we looked over the Yupno River meandering toward the Bismarck Sea. “Let's go over that one more time,” we suggested.

Danda obliged, again using his hands to enliven his explanation. But as we expected, his gestures had changed. As he referred to “yesterday,” he now gestured, not backward, but forward. As he explained “tomorrow,” he gestured back over his shoulder, up toward the ridge [see illustration below]. Inconsistent as these movements may seem, Danda was not confused. His gestures expressed the Yupno way of understanding time, one in which the future is not something in front of you—it is uphill. By having interviewees change sitting positions, we were able to show that it does not matter whether the slope is in front of you, behind you, to your left or to your right. The Yupno conception of time is not anchored to the body, as the Western one is, but to the world and its contours. By investigating cases such as these, we and other researchers are starting to piece together an answer to a question that has puzzled thinkers for centuries: How are human beings able to make sense of time?

Humans, like creatures ranging from amoebas and bees to mockingbirds and elephants, come with built-in equipment for perceiving some aspects of time, such as the rhythms of night and day, the waxing and waning of the moon, and the turning of the seasons. What separates humans from other animals is that we do not stop at merely sensing time's passage. We tackle time head-on—or at least we try. We dice it into units, even ones that go beyond what is perceivable, such as milliseconds, or that transcend our life span, such as millennia. We depict time graphically, talk about it ceaselessly and even make gestural models of it in the air as we talk. In short, humans everywhere create and rely on time concepts—ideas about the nature of time that allow us to make plans, follow recipes, share memories and discuss possible futures.

A Yupno man produces contrasting gestures when talking about time. When he faces uphill (1), yesterday is behind him. When he faces downhill (2), yesterday is in front of him. His gestures for “tomorrow” (3 and 4) reflect the opposite pattern. These gestures express the Yupno way of thinking about the past as downhill and the future as uphill. Credit: “CONTOURS OF TIME: TOPOGRAPHIC CONSTRUALS OF PAST, PRESENT, AND FUTURE IN THE YUPNO VALLEY OF PAPUA NEW GUINEA,” BY RAFAEL NÚÑEZ ET AL., IN COGNITION, VOL. 124, NO. 1; JULY 2012

But what are our time concepts made of? What is going on in the mind of a speaker of Yupno, or English for that matter, when answering our question about the difference between yesterday and tomorrow? Recent research in cognitive science is uncovering a surprising answer. Across cultures, human time concepts depend, in large part, on metaphor—in particular, on what cognitive scientists call conceptual metaphor, in which we think about something, in this case time, in terms of something else, in this case space. Thus, we build our understanding of duration, of time's passage and of sequences of events out of familiar spatial ideas such as size, movement and location. The latest findings reveal that this basic “time is like space” metaphor appears to be universal around the world—yet it also takes strikingly different forms from one culture to the next.

The “Time Is Like Space” Metaphor

The puzzle of how humans are able to understand time is an ancient one. Philosopher Saint Augustine put his finger on it in A.D. 400 when he commented: “Who can even form a conception of it to be put in words? Yet what do we mention more often or familiarly in our conversation than time?” Time is slippery and ethereal, but we nonetheless have to grapple with it every day. Only in the past century have researchers started to study human time concepts with an empirical eye, and they started by looking closely at the language people use to talk about time. Benjamin Lee Whorf, famous for his idea that the language you speak guides the way you think, keenly observed in the early 20th century that speakers of English and many other European languages talk of time as “motion on a space” and imagine time units as standing “in a row.”

Whorf also claimed that the Hopi, a Native American tribe, conceived of time in their language without such spatial metaphors. Later researchers showed that spatial metaphors for time are actually rampant in Hopi, just as they are in English. More remarkably, it turns out that all human cultures seem to treat time through spatial metaphors (although these metaphors are more or less pronounced across languages). Durations are talked about using words for size (“a short weekend”). Time's passage is treated as movement (“the week flew by”). Events are imagined as located at different positions on a path, in two different ways—by taking an internal perspective or an external one [see illustration below].

We sometimes imagine ourselves inside the sequence of events, with past, present and future conceptualized as locations where we once were, currently are and will be. This internal perspective on time motivates English expressions such as “the week ahead of us.” When we take the external perspective, however, we view the succession of events from the outside, much like watching a lineup of people all moving in one direction. This external perspective motivates phrases such as “a reception follows the ceremony.”

These basic ideas about time are expressed spatially in a dazzling variety of unrelated languages, across cultures that differ in every way imaginable. The idea that temporal sequences are like queues of people is found, for example, in Tamil (India), Maori (New Zealand), Greenlandic (Greenland) and Sesotho (South Africa), where the idea that “spring follows winter” can be expressed as “spring is in the footprints left by winter.”

Credit: ISTOCK.COM (walking man, head, uphill and sun icons); ROYAL ASTRONOMICAL SOCIETY Science Source (Mandarin text); SCIENTIFIC AMERICAN MIND (globe)

But now we come to a wrinkle. Even as people of all cultures lean on spatial concepts for understanding time, exactly which spatial metaphors they use can vary. Take the internal perspective, future-in-front metaphor mentioned earlier, found in English and many other languages. This metaphor was long thought to be universal, but in 2006 members of our team investigated a striking counterexample in South America. In Aymara, a language spoken high in the Andes, many phrases suggest the opposite metaphor is at work. For example, the expression “a long time ago” could be loosely rendered in Aymara as “a lot of time in front.” Analysis of video-recorded interviews with 30 speakers showed conclusively that Aymara speakers gesture according to this future-behind, past-in-front metaphor [see “Where the Future Is Behind” below]. The pattern is especially strong among older speakers who do not speak Spanish, which has the future-in-front metaphor common to English and most European languages.

Elsewhere the divisions between past, present and future are not made according to divisions of the human body at all. In Pormpuraaw, an Aboriginal Australian community, past and future are determined by cardinal directions, with past times to the east and future times to the west. In Yupno, as Danda's gestures made clear, the future is uphill and the past downhill. These different spatial metaphors reflect different cultural preoccupations. Yupno speakers, for example, prefer to carve up the world in terms of uphill and downhill—not completely surprising given the mountainous landscape they call home. They use slopes as points of reference even when this might seem odd to Westerners, such as when inside flat, windowless houses.

Forward in Time, Forward in the Mind

Although all cultures make use of time-as-space metaphors, a skeptic could counter that spatial metaphors might merely be used for communicating about time. Maybe in the privacy of our own minds, metaphors fall away. In fact, people draw on space when reasoning about time in all kinds of situations and even when all by themselves.

One line of research has demonstrated this by showing that adults—and even young children—are simply not able to ignore cues from space when asked to judge something about time. For example, when study subjects estimate how long a line was present on a computer screen, the actual length of the depicted line influences how “long” they think the line was visible: longer lines seem to have lingered longer. Other studies have shown that when you ask people to think about time, it can interfere with how they do simple spatial tasks. A 2016 study conducted in Italy asked 19 participants to categorize words such as “yesterday” or “tomorrow” as relating to either the past or the future. Sometimes subjects had to do this categorization by stepping forward when they heard a future word and backward when hearing a past word—consistent with their native metaphor. But other times subjects had to do the opposite, stepping backward for future and forward for past. In these cases, people were significantly slower to step and made about twice as many errors in categorizing the words. Our team recently obtained similar results with English speakers.

A particularly telling line of research is based on a seemingly simple question: “Next Wednesday's meeting has been moved forward two days. What day will the meeting be on?” Ask a group of people this question, and about half will say Friday and the other half will say Monday. “Forward” is ambiguous when used to refer to time, at least in English. If, using an internal perspective, you imagine next week as something ahead that you are moving toward, then “forward” would mean further into the future, and thus you should respond Friday. But if you externalize time and imagine next week as a lineup of days, with earlier days toward the front of the line, then “forward” would mean earlier in the sequence, and thus you should respond Monday.

Researchers have used this question in more than a dozen experiments, to learn more about how spatial experiences influence thinking about time. In one study, researchers at Stanford University posed the question to more than 300 people at San Francisco International Airport. They found that people who had just gotten off the plane chose more Friday answers when compared with people who were at the airport just to pick someone else up. The experience of moving forward in actual space primed the travelers to imagine “moving forward” in time. In fact, people who were about to get on the plane and were thus merely imagining forward motion also tended to answer Friday. It is also possible to prime people to respond Monday. In one of our studies, people who first answered questions about a lineup of boxes tended to think about time as an external sequence. Forward is earlier in a sequence, so they favored Monday answers.

These findings raise the possibility that parts of the brain used for thinking about space may also be used for thinking about time. Now a handful of studies have provided the first direct evidence for this. In one, published in 2015, researchers in Belgium scanned the brains of participants as they answered questions about the order of events that had recently happened or were about to happen. The researchers found that performing this task engaged a network of brain areas known to support spatial imagery. Another study found that people with damage to space-related brain areas also have trouble thinking about time. When we take an external perspective on time, in English and many European languages, we often imagine past events to the left and future events to the right. Accordingly, the study found that people with disrupted spatial abilities on their left side had trouble remembering past events and even confused past events for future ones.

The Influence of Writing

The time-as-space metaphor shows up in our language and gestures, and it is active in our minds even when we are not communicating about time. It also shows up with striking clarity in depictions of external sequences of events, which have come to saturate every aspect of our visual culture. Histories are laid out on timelines. T-shirts riff on the iconic image of human evolution as proceeding rightward, creature by creature. No matter whether your calendar is printed or digital, it no doubt shows the days arranged from left to right and the weeks from top to bottom.

Using spatial metaphors, we imagine time in two ways: as a path we walk with future events in front and past behind (left) or as a sequence we view externally, as in summer, fall, winter, spring (right). Credit: SOURCE: “THE TANGLE OF SPACE AND TIME IN HUMAN COGNITION,” BY RAFAEL NÚÑEZ AND KENSY COOPERRIDER, IN TRENDS IN COGNITIVE SCIENCES, VOL. 17, NO. 5; MAY 2013; ISTOCK.COM (woman and man icons)

The most subtle representation of time unfolding is written text, and it may also be the most powerful. Regardless of what script you are using, the symbols are presented in linear order. For instance, Latin script, as used in English, proceeds from left to right—thus earlier symbols are to the left of later symbols—and this guides intuitions about which way time flows. If you have people arrange three temporally ordered images—a banana in its peel, the banana partially peeled and the banana half-eaten—English speakers will lay them out from left to right, but Hebrew speakers will lay them out from right to left, echoing how Hebrew is written. Mandarin Chinese speakers refer to “last week” as above and “next week” as below, which stems from the fact that Mandarin is traditionally written vertically, from top to bottom. Moreover, blind people develop a left-to-right model of time based solely on their experience reading Braille, which flows from left to right.

Writing is so powerful, in fact, that one study found that it is possible to reverse time's imagined “direction” by training people to read writing that had been mirror-reversed. The researchers had Dutch participants categorize words as being related to the past or the future, much like the stepping study described earlier but with button presses instead of steps. In general, people were faster to categorize past events by pressing a button on the left and future movements by pressing a button on the right. But after just a few minutes of reading mirrored text, they showed the opposite pattern. Graphical practices, where they are used, can thus shape our conceptions of time in powerful ways. Reading and writing, however, are not needed to develop the intuition that time can be thought of spatially, as evidenced by Aymara, Yupno and other cultures that lack writing traditions but that nonetheless spatialize time. Such cultures offer clues about how time has been grasped since before the invention of writing.

The Future of Time

Time may be the abstract concept we use the most—in the words of one scholar, J. T. Fraser, it is a “familiar stranger.” This makes time a critical case study for understanding the human capacity for abstract thinking, a hallmark of our species. But time is not the only abstract domain, nor is it the only one we understand through metaphor. For some ethereal notions, we use metaphors based on color, weight, heat or even sound—such as when we feel heavy-hearted or boiling mad. But spatial metaphors are particularly common, structuring how we think about not only time but also causation and kinship, politics and power (as in the phrase, “she has the upper hand”). Why is space at the root of so many metaphors? Space is something humans have evolved to be especially good at navigating, and our brain has an elaborate system for mapping it. When we think about, talk about and depict abstract ideas as spatial in nature, we are capitalizing on hard-won cognitive talents.

The human reliance on spatial metaphors for abstract thinking may have deep evolutionary roots and is not likely to change any time soon. The particular metaphors we lean on, however, are a product of culture—not of biological evolution—and are much more malleable. Literacy is a recent and rapid achievement in the scope of the human saga, but it already has had profound consequences for how people conceptualize time. New spatial metaphors for our dearest abstract concepts will almost certainly enter the picture as our culture evolves. E-mail in-boxes show the most recent items at the top, but text messages go the other way, with the newest at the bottom. And so we must wonder: Which way will time flow next?

Where the Future is Behind


Speakers of Aymara, a language of the Andes, conceive of the past as being in front of them and the future as behind them. The pattern can be seen in everyday language. Nayra pacha means “old times,” where nayra is the word for “eye” or “front,” and pacha roughly means “time” or “epoch.” Qhipa marana means “next year,” where qhipa is the word for “behind,” and mara is “year.” This conception, however, is not just a matter of words. It is also observable in the spontaneous gestures Aymara speakers produce while talking, often pointing backward when referring to future times and forward when discussing the past. What motivates this “reversed” pattern?

For the Aymara, knowledge acquired through visual perception is taken to be certain and reliable. It is of utmost importance to communicate facts and stories by grammatically marking whether what is being said has been seen directly or learned from another source. Aymara people spend much more time talking about past events than about future times. After all, they can tell whether last year was dry or wet—they were there and saw it with their eyes—and can discuss that with clarity and conviction. But how next year is going to be is anybody's guess—nobody has seen it, and so it is just a matter of futile speculation. The known past is therefore conceived as being visually in front of them and the unknown future out of view behind them.