That ordinary scene in central New Jersey would not have looked at all ordinary to an American from colonial times—say, one of the patriots who helped George Washington win the Battle of Trenton in 1776. At the time, Trenton was a small village, and farms lined the route of Washington’s epic night march from Trenton to Princeton—a march that took him right past the future site of the giant Quakerbridge shopping mall.

Imagine that you could transport an American from the colonial period forward in time to our own era. (Isn’t that the plot of a movie? Several, actually.) What would this time-traveler find amazing?

Surely the most amazing thing would be the sheer prosperity of modern America—the range of goods and services that ordinary families can afford. Looking at all that wealth, our transplanted colonial would wonder, “How can I get some of that?” Or perhaps he would ask himself, “How can my society get some of that?”

The answer is that to get this kind of prosperity, you need a well-functioning system for coordinating productive activities—the activities that create the goods and services people want and get them to the people who want them. That kind of system is what we mean when we talk about the economy. And economics is the social science that studies the production, distribution, and consumption of goods and services.

An economy succeeds to the extent that it, literally, delivers the goods. A time-traveler from the eighteenth century—or even from 1950—would be amazed at how many goods and services the modern American economy delivers and at how many people can afford them. Compared with any past economy and with all but a few other countries today, America has an incredibly high standard of living.

So our economy must be doing something right, and the time-traveler might want to compliment the person in charge. But guess what? There isn’t anyone in charge. The United States has a market economy, in which production and consumption are the result of decentralized decisions by many firms and individuals. There is no central authority telling people what to produce or where to ship it. Each individual producer makes what he or she thinks will be most profitable; each consumer buys what he or she chooses.

The alternative to a market economy is a command economy, in which there is a central authority making decisions about production and consumption. Command economies have been tried, most notably in the former Soviet Union between 1917 and 1991. But they didn’t work very well. Producers in the Soviet Union routinely found themselves unable to produce because they did not have crucial raw materials, or they succeeded in producing but then found that nobody wanted their products. Consumers were often unable to find necessary items— command economies are famous for long lines at shops.

Market economies, however, are able to coordinate even highly complex activities and to reliably provide consumers with the goods and services they want. Indeed, people quite casually trust their lives to the market system: residents of any major city would starve in days if the unplanned yet somehow orderly actions of thousands of businesses did not deliver a steady supply of food. Surprisingly, the unplanned “chaos” of a market economy turns out to be far more orderly than the “planning” of a command economy.

In 1776, in a famous passage in his book The Wealth of Nations, the pioneering Scottish economist Adam Smith wrote about how individuals, in pursuing their own interests, often end up serving the interests of society as a whole. Of a businessman whose pursuit of profit makes the nation wealthier, Smith wrote: “[H]e intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention.” Ever since, economists have used the term invisible hand to refer to the way a market economy manages to harness the power of self-interest for the good of society.

The study of how individuals make decisions and how these decisions interact is called microeconomics. One of the key themes in microeconomics is the validity of Adam Smith’s insight: individuals pursuing their own interests often do promote the interests of society as a whole.

So part of the answer to our time-traveler’s question—“How can my society achieve the kind of prosperity you take for granted?”—is that his society should learn to appreciate the virtues of a market economy and the power of the invisible hand.

But the invisible hand isn’t always our friend. It’s also important to understand when and why the individual pursuit of self-interest can lead to counterproductive behavior.