For more than 99 percent of our evolutionary history, we lived as foragers, and our recent ancestors must have 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 nonliterate foragers do with thier 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 one without a Swiss Army knives and freeze-dried pasta.

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, concealed pits, and clifftops, blowguns, and bows and arrows. Animal prey may thus be flushed out, cracked open, trapped, ambushed, or done in with weapons. Plants are cut down or unearthed, shelled and skinned, and detoxified by cooking, soaking, fermenting, and other tricks of the kitchen magician.

How do they accomplish these feats? With the help of language, foragers pool their knowledge and coordinate their actions. Their words offer a window to the kinds of knowledge they possess. All documented human cultures (and by extrapolation, ancestral foraging ones) have words for space, time, motion, number, mental states, tools, flora, fauna, and weather; they make logical distinctions between general and particular, apparent and real, possible and actual. People use words and concepts to reason about invisible entities such as disease, meteorological forces, and absent animals. They also possess knowledge that is not easily expressed in words. 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 we humans evolved mental machinery that allowed us to cooperate and outsmart the local flora and fauna. Vital to that machinary is its ability to analyze and categorize experiences that reflect the world's causal structure, which lets us make good predictions about unseen events. The world is a heterogeneous place, with many kinds of entities--and laws that make them tick. The mental toolbox we inherited comes equipped with distinct kinds of intuitions--about space, number, objects living things, tools, and minds. We can see these intuitions at work as children first try to make sense of their world.

The most fundamental mental tool is an intuitive physics: an understanding of 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.

Living things are sensed to house an internal essence, which supplies a source of renewable energy,or oomph, that propel animals (usually in pursuit of a goal), gives them their form, and drives their growth and bodily functions.

This intuition guides the way people in all cultures treat the living world. Foragers are fine amatuer biologists who classify local plants and animals into categories that often match the professional biologist's genus or specis. They don't simply lump together animals that look alike; dissimilar-looking creatures such as caterpillars and butterflies or peacocks and peahens are classified as the same animal. The intuition that organisms are driven by an internal constitution also allows foragers to predict movements and life cycles. Straight tracks tell of a beast aiming for a spring may provide fruit or a nutritious underground tuber in the fall. The same intuition inspires foragers to prepare juices and powders from plants and animals and try them out as medicines, poisons, and food additives.

A third way of knowing is intuitive engineering, the understanding of tools and other artifacts. Artifacts are defined not by their shape or constitution but by what they are intended to do. A store selling chairs might be stocked with anything from stools and dinning room sets to beanbags, hammocks, and wooden cubes. A stump or elephant's foot becomes a chair if someone decides to use it as one. The only thing chairs have in common is that someone intends them to hold up a human behind.

No law of intuitive physics, biology, or engineering, however, can explain the actions of human beings, who clearly don't behave like rocks, animals, or wind-up dolls. To predict the deeds of others, 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 can make good guesses by listening carefully to what they say, watching their faces and eyes, and trying to make sense of their behavior.

A child's precocious understanding of these domains--psychology, biology, physics, and engineering--suggests that the brain is prepared for them.

According to a saying that is well known among psychologists, if you give a boy a hammer, the whole world becomes a nail. The saying is usually aimed at overreaching theorecticians, but it seems to be appropriate to Homo sapiens in general. If you give a species an elementary grasp of psychcology, biology, and mechanics, then for better and for worse, the whole world becomes a society, azoo, and a machine.