There is no mystery to what an economy is. Whether we are talking about the economy of Los Angeles, the United States, or the whole world, an economy is just a group of people dealing with one another as they go about their lives. Because the behavior of an economy reflects the behavior of the individuals who make up the economy, we begin our study of economics with four principles of individual decision making.
Principle 1: People Face Trade-offs
Principle 2: The Cost of Something Is What You Give Up to Get It
Principle 3: Rational People Think at the Margin
Principle 4: People Respond to Incentives