The Brain’s Versatile Toolbox

Steven Pinker

(Adapted from How the Mind Works)

The human brain is an extraordinary organ. It has allowed us to walk on the moon, *to discover the of matter and life,* and to play chess almost as well as a computer. But this virtuosity raises a puzzle. The brain of Homo sapiens achieved its modern form and size between fifty and a hundred thousand years ago, well before the invention of agriculture, civilizations, and writing in the last ten thousand years. Our foraging ancestors had no occasions to do astrophysics or play chess, and natural selection would not have rewarded them with more babies if they had. How, then, did our outsize, *science-ready* brain evolve?

This puzzle led Alfred Russel Wallace, the co-discoverer of natural selection, to defect to creationism, and *ever since* it has frustrated our attempts to understand the mind as part of the natural world. But the puzzle can be solved with a key idea: the process of natural selection * equipped our ancestors with a mental toolbox of intuitive theories about the world, *which they used to master rocks, tools, plants, animals, and one another. We use the same toolbox today to handle the intellectual challenges of modern societies, including the most abstruse concepts of science and mathematics.*

For more than 99 percent of our evolutionary history, our ancestors lived as foragers, and it seems safe to assume that *as they evolved into us, they lived much as foraging tribes do today, without any of the trappings of modern civilization. Observing their seemingly simple life, many people have wondered what preliterate foragers do with their capacity for abstract intelligence. * The foragers would have better grounds for asking that question about modern couch potatoes. A foraging life is a camping trip that never ends, but without the Swiss Army knives and freeze-dried pasta. *Living by their wits, human groups develop sophisticated technologies and bodies of folk science.*

All foraging peoples use fire and shelters and manufacture many kinds of tools. Their engineering is often ingenious, involving poisons, smokeouts, glue traps, nets, baits, snares, corrals, camouflaged pits and clifftops, blowguns, bows and arrows, and kites trailing fishing lines made of sticky spider silk. Animal prey may be flushed out, cracked open, trapped, or ambushed. Plants are cut down and unearthed, shelled and skinned, and detoxified by cooking, soaking, fermenting, leaching and other tricks of the kitchen magician; some plants are transformed into medicinal drugs.

With the help of language, foragers pool their knowledge and coordinate their actions. All documented human cultures (and by extrapolation, ancestral foraging ones) have words for space, time, motion, number, mental states, tools, flora, fauna, and weather and make logical distinctions between general and particular, apparent and real, possible and actual. People use these words and concepts to reason about invisible entities like disease, meteorological forces, and absent animals. Their mental maps may contain thousands of noteworthy sites, and their mental calendars record cycles of weather, animal migrations, and the life histories of plants.

So humans evolved mental machinery that allowed us to cooperate and outsmart the local flora and fauna. Vital to that machinery--what makes it so powerful and essential to foragers and neuroscientists alike--is its ability to analyze the world into useful categories. The world is a heterogeneous place. To generalize from our experiences properly and make good predictions about events unseen, we need to understand something of the causal structure of the world--its contents and the laws that make it tick. Thanks to our ancestors' hard-won mental toolbox, we seem to be endowed with several kinds of intuitions that do just that.

The most fundamental mental tool is an intuitive physics: understanding how objects fall, roll, and bounce. Its foundation is an appreciation that the world contains objects that persist when out of sight and that obey laws; it is not a kaleidoscope of shimmering pixels or a magic show in which things disappear and reappear capriciously. Philosopher and psychologist William James described the world of the infant as a "blooming, buzzing confusion," but recent (?) experiments have shown that babies are not as confused as James thought. Infants as young as three months are visibly surprised when an experimenter rigs up a display in which objects seem to vanish, pass through each other, fly apart, or move without having been pushed. As one psychologist summed up the results, "a blooming, buzzing confusion" is a good description of the life of the parents, not the infant, who is perfectly able to interpret all the blooms and buzzes as outward signs of persisting, law-abiding objects.

But some objects do seem to defy physical laws. As evolutionary biologist Richard Dawkins noted, if you throw a dead bird into the air, it will describe a graceful parabola and come to rest on the ground, exactly as physics books say it should, but if you throw a live bird in the air it may not touch land this side of the county boundary. These apparent scofflaws are living things, and we interpret them not as weird springy objects, nor as law-defying miracles, but as obeying a different kind of law, the laws of an intuitive biology. Living things are sensed to house an internal essence, which supplies a renewable source of energy or oomph that propels the animal (usually in pursuit of a goal), gives it its form, and drives its growth and bodily functions.

This intuition guides the way people in all cultures treat the living world. Foragers are fine amateur biologists who classify local plants and animals into categories that often match the professional biologist's genus or species. The intuition that organisms are driven by an internal constitution also allows foragers to predict their movements and life cycles. Straight tracks tell of a beast aiming for a destination, at which it can perhaps be surprised; a flower in the spring may provide fruit or a nutritious underground tuber in the fall. The same intuition inspires foragers to try out plant and animal parts as medicines, poisons, and food additives,

Children distinguish the living from the nonliving early in life. Infants expect objects to move only when launched by a collision, but expect people to start and stop on their own. Preschoolers reason about animals by ignoring appearances and focusing on their innards. When asked what would happen of you removed the insides of a dog, leaving a shell that looks like a dog, children say it is not a dog and can't bark or eat dogfood. But when asked what would happen if you removed the outsides of a dog, leaving something that doesn't look like a dog at all, they say it's still a dog and does doggy things.

A third way of knowing is intuitive engineering, the understanding of tools and other artifacts. Tools appear in the fossil record millions of years before modern skulls do and must have been a major selection pressure for the expansion of the brains that make them. Today's one-year-old hominids tinker with sticks for pushing, strings for pulling, and supports for holding things up. Before they enter first grade, children have different intuitions about artifacts and living things. If you make a lion look like a tiger with costumes or surgery, children say it is not a tiger but still a lion. But if you make a coffeepot look like a birdfeeder, they say it just is a birdfeeder.

These children are aware that artifacts are defined not by their shape or constitution but by what someone fashioned them to do. A store selling "chairs" might be stocked with anything from stools and dining room sets to beanbags, hammocks, foam cylinders, and wooden cubes. A stump or elephant's foot becomes a chair if someone decides to use it as one. The only thing that "chairs" have in common is that someone intends them to hold up a human behind.

No law of physics, biology, or engineering, however, can explain, or predict, human behavior. For that we need intuitive psychology--the conviction that people are driven by invisible, weightless mental states such as beliefs and desires. We mortals can't literally read other people's minds, but we make good guesses--by listening to what they say, reading between the lines, watching their face and eyes, and trying to make sense of their behavior. Like the other core intuitions, the rudiments of mind reading are first exercised in the crib. Infants make eye contact and track their parents' gaze, especially then they are uncertain why a parent is doing something. Three-year-olds know that a looker generally wants what he is looking at, that you can't eat the memory of an apple, and that a person can tell what's in a box only by looking in it.

A child's precocious understanding of these four domains--psychology, biology, physics, and engineering--suggests that the brain is prepared for them. Indeed, some patients with brain damage cannot name living things but can name artifacts, or vice versa, implying that artifacts and living things are stored in different ways in the brain. And some kinds of mental disorders seem to impair some domains and leave others spared. People with autism, for example, seem to lack an intuitive psychology, whereas those with Williams Syndrome are competent intuitive psychologists but are spatially and mechanically challenged.

Our mental tools are sometimes most conspicuous when we apply them in ways they are not designed for. Much slapstick humor comes from a sudden shift away from thinking of a person in the usual way, as a living locus of beliefs and desires, to seeing him as a material object ignominiously obeying the laws of physics (such as slipping on a banana peel). Religious beliefs in souls, angels, and gods come from divorcing our intuitive psychology from our intuitive biology and physics so that we can think about minds that have no bodies. Animistic beliefs do the opposite--they marry intuitive psychology to intuitive biology, physics, or engineering and impute minds to trees, mountains, or idols.

And this brings us back to how stone age minds grasp modern science. Formal sciences grew out of their intuitive counterparts. The conviction that living things have an essence, for example, is what impelled the first professional biologists to try to understand the nature of plants and animals by cutting them open and putting bits of them under a microscope. Anyone who announced he was trying to understand the nature of chairs by bringing them into a laboratory and putting bits of them under a scope would be dismissed as mad, not given a grant.

But modern science forces us to make some changes in our thinking, including turning offparts of the intuitions out of which it grew. Newton's first law states that a moving object continues in a straight line unless acted on by a force. Ask college students what happens to a whirling tetherball that is cut loose, however, and a depressingly large minority, including many who have taken physics, say it would continue in a circular path. The students explain that the object acquires a "force" or "momentum" that powers it along the curve until the momentum gets "used up" and the path straightens out. Although erroneous, the students' beliefs are completely understandable since we evolved in a world with substantial friction that makes moving objects slow down and stop, not in a lab with pucks gliding on air tables.

Modern science also pries our intuitive faculties loose from the objects they usually apply to and aims them at seemingly inappropriate ones. To do mathematics, we primates--visual animals--invented graphs. These allow abstruse concepts to present themselves to our mind's eyes as reassuringly familiar shapes: "Y=mx+b" is a straight line, differentiable functions are smooth curves. They also allow mathematical operations to be performed by doodling in mental imagery: to add a constant, mentally shove the line upward; to multiply, rotate it; to integrate, color in the space beneath it. To do chemistry, we stretch our intuitive physics and treat the essence of a natural substance as a collection of tiny, bouncy, sticky objects. To do biology, we take our way of understanding artifacts and apply it to living things--organs as machines "engineered" by natural selection--and then to their essences, the molecule of life. To do psychology, we treat the mind as an organ of a living creature, as an artifact designed by natural selection, and as a collection of physical objects, neurons.

According to a saying, if you give a boy a hammer, the whole world becomes a nail. If you give a species an elementary grasp of psychology, biology, and mechanics, then for better and worse, the whole world becomes a society, a zoo, and a machine.