Every field of study has its own language and its own way of thinking. Mathematicians talk about axioms, integrals, and vector spaces. Psychologists talk about ego, id, and cognitive dissonance. Lawyers talk about venue, torts, and promissory estoppel.

每个研究领域都有它自己的语言和思考方式。数学家谈论定理、积分以及向量空间。心理学家谈论自我、本能、以及认知的不一致性。律师谈论犯罪地点、侵权行为以及约定的禁止翻供。

Economics is no different. Supply, demand, elasticity, comparative advantage, consumer surplus, deadweight loss—these terms are part of the economist’s language. At first, this new language may seem needlessly arcane. But, as you will see, its value lies in its ability to provide you a new and useful way of thinking about the world in which you live.

经济学家也一样。供给、需求、弹性、比较优势、消费者剩余、最低损耗，这些术语是经济学语言的一部分。起初，这些新的语言可能看起来不必要的神秘，但是，你会看到，他的价值在于他能提供给你一种新的和有用的思考方式，让你思考你生活的世界。

Economists try to address their subject with a scientist’s objectivity. They approach the study of the economy in much the same way as a physicist approaches the study of matter and a biologist approaches the study of life: They devise theories, collect data, and then analyze these data in an attempt to verify or refute their theories.

经济学家们试图从科学的角度来阐释他们的学科。他们演绎经济学和物理学家演绎物质、生物学家演绎生命一样：他们设计理论，收集数据，然后分析这些数据，试图能够证明或者反驳他们的理论。

To beginners, it can seem odd to claim that economics is a science. After all, economists do not work with test tubes or telescopes. The essence of science, however, is the scientific method—the dispassionate development and testing of theories about how the world works. This method of inquiry is as applicable to studying a nation’s economy as it is to studying the earth’s gravity or a species’ evolution. As Albert Einstein once put it, “The whole of science is nothing more than the refinement of everyday thinking.”

对于初学者来说，宣称经济学是科学似乎看起来很奇怪。毕竟，经济学家们并不和试管或者显微镜一起工作。然而，科学的精华是科学的方法——世界如何运作这一理论的逐渐发展和试验。这种观察的方法运用于研究一个国家的经济，就像它运用于研究地球的重力或者一个物种的演变。就像爱因斯坦曾经提出的：科学的全部无非就是日常思考的精炼。

Although Einstein’s comment is as true for social sciences such as economics as it is for natural sciences such as physics, most people are not accustomed to looking at society through the eyes of a scientist. Let’s therefore discuss some of the ways in which economists apply the logic of science to examine how an economy works.

尽管爱因斯坦的观点对于社会科学例如经济学是正确的，就像它对于自然科学例如物理学一样是正确的，大部分人并不习惯于通过科学的角度来看待社会。因此，就让我们来讨论一些方法，通过这些方法，经济学家们运用科学逻辑来观察经济如何运作。

Isaac Newton, the famous seventeenth-century scientist and mathematician, allegedly became intrigued one day when he saw an apple fall from an apple tree. This observation motivated Newton to develop a theory of gravity that applies not only to an apple falling to the earth but to any two objects in the universe. Subsequent testing of Newton’s theory has shown that it works well in many circumstances (although, as Einstein would later emphasize, not in all circumstances). Because Newton’s theory has been so successful at explaining observation, it is still taught today in undergraduate physics courses around the world.

牛顿，17世纪著名的科学家和数学家，当有一天看到一个苹果从树上掉下时，引起了他的深思。这个观察使牛顿形成了重力的理论，这一理论不仅应用于苹果落地，而且应用于宇宙中的任何两个物体之间。牛顿定律的不断试验表明，它适用于很多情况（尽管爱因斯坦后来强调并不是适用于所有的情况）。由于牛顿定律在解释试验时很成功，所以今天它仍然在世界各个大学的物理课程中被学习。

This interplay between theory and observation also occurs in the field of economics. An economist might live in a country experiencing rapid increases in prices and be moved by this observation to develop a theory of inflation. The theory might assert that high inflation arises when the government prints too much money. (As you may recall, this was one of the Ten Principles of Economics in Chapter 1.) To test this theory, the economist could collect and analyze data on prices and money from many different countries. If growth in the quantity of money were not at all related to the rate at which prices are rising, the economist would start to doubt the validity of his theory of inflation. If money growth and inflation were strongly correlated in international data, as in fact they are, the economist would become more confident in his theory.

现象和理论的这一关系也同样发生在经济领域。经济学家可能生活在这样的国家，这个国家的物价正在迅速增长，在这一现象的推动下，经济学家发现了关于通货膨胀的理论。这一理论可能宣称：当政府印制太多货币时，高的通货膨胀就会出现（你可能会想起，这是第一章中十条经济学原理之一）。为了验证这一理论，经济学家会从不同国家收集并分析关于价格和货币量的数据。如果货币量的增长和价格膨胀率根本不相关，经济学家就会开始怀疑通货膨胀理论的正确性。如果数据表明货币增长量和通胀高度相关，经济学家就会对他的理论更加有信心。

Although economists use theory and observation like other scientists, they do face an obstacle that makes their task especially challenging: Experiments are often difficult in economics. Physicists studying gravity can drop many objects in their laboratories to generate data to test their theories. By contrast, economists studying inflation are not allowed to manipulate a nation’s monetary policy simply to generate useful data. Economists, like astronomers and evolutionary biologists, usually have to make do with whatever data the world happens to give them.

尽管经济学家像其他科学家一样应用理论和现象，他们确实面临一个障碍：在经济学中做试验往往很困难，这一障碍使他们的工作更具挑战性。物理学家研究向心力可以在实验室里降落许多物体来产生数据以测试他们的理论。相比之下，经济学家研究通货膨胀不可以仅仅为了产生有用的数据，而去操纵一个国家的货币政策。像宇宙学家和生物学家一样，一般来说经济学家不得不运用世界已经发生的数据来做试验。

To find a substitute for laboratory experiments, economists pay close attention to the natural experiments offered by history. When a war in the Middle East interrupts the flow of crude oil, for instance, oil prices skyrocket around the world. For consumers of oil and oil products, such an event depresses living standards. For economic policymakers, it poses a difficult choice about how best to respond. But for economic scientists, it provides an opportunity to study the effects of a key natural resource on the world’s economies, and this opportunity persists long after the wartime increase in oil prices is over.为了找到实验室试验的替代实验，经济学家密切关注于由历史提供的自然实验。例如，当中东的一次战争打断了原油的供应，石油价格在全球范围狂涨。对于石油及石油生产品的消费者来说，这样的一次事件降低了生活水平。对于经济决策者，如何让应对这样的事件，给他们出了一道难题。但是对于经济科学家来说，它为重要自然资源对于世界经济的影响的研究提供了一次机会，这次机会持续很久，直到石油价格增长的战争时间结束。

If you ask a physicist how long it would take for a marble to fall from the top of a ten-story building, she will answer the question by assuming that the marble falls in a vacuum. Of course, this assumption is false. In fact, the building is surrounded by air, which exerts friction on the falling marble and slows it down. Yet the physicist will correctly point out that friction on the marble is so small that its effect is negligible. Assuming the marble falls in a vacuum greatly simplifies the problem without substantially affecting the answer.

如果你问一个物理学家一块大理石从十层高的大楼顶层落下，需要多长时间，她回答这个问题会假定大理石在真空中落下。当然，这个假设是错误的。事实上，这个大楼被空气环绕，空气会对大理石降落产生摩擦力，并减慢他的速度。然而物理学家会指出：空气对大理石的摩擦力太小以致它的影响可以忽略。假定大理石在真空中降落，并且几乎没有影响答案，大大简化了问题。

Economists make assumptions for the same reason: Assumptions can make the world easier to understand. To study the effects of international trade, for example, we may assume that the world consists of only two countries and that each country produces only two goods. Of course, the real world consists of dozens of countries, each of which produces thousands of different types of goods. But by assuming two countries and two goods, we can focus our thinking. Once we understand international trade in an imaginary world with two countries and two goods, we are in a better position to understand international trade in the more complex world in which we live.

经济学家作假设基于相同的原因：假设能够使世界更容易理解。例如，为了研究国际交易的影响，我们可以假设世界仅有两个国家，并且每个国家仅仅生产两种产品。当然，现实的世界有很多国家组成，每个国家生产上千种不同的产品。但是通过假设两个国家和两种产品，我们可以更好的思考。一旦我们理解了假设中的两个国家两种产品的国际交易，那么我们可以更好地理解我们生活的这一更加复杂的世界的国际交易。

Economists use different assumptions to answer different questions. Suppose that we want to study what happens to the economy when the government changes the number of dollars in circulation. An important piece of this analysis, it turns out, is how prices respond. Many prices in the economy change infrequently; the newsstand prices of magazines, for instance, are changed only every few years. Knowing this fact may lead us to make different assumptions when studying the effects of the policy change over different time horizons. For studying the short-run effects of the policy, we may assume that prices do not change much. We may even make the extreme and artificial assumption that all prices are completely fixed. For studying the long-run effects of the policy, however, we may assume that all prices are completely flexible. Just as a physicist uses different assumptions when studying falling marbles and falling beach balls, economists use different assumptions when studying the short-run and long-run effects of a change in the quantity of money.

相似地，经济学家用不同的假设来回答不同的问题。假设我们要研究当政府改变流通中的货币时，会对经济产生什么样的影响。结果表明，这个分析很重要的一条是价格是如何回应的。经济中很多价格会经常变动，报摊上的杂志价格变动会隔几年。懂得这一事实，可以让我们在研究不同时期政策改变的影响时，做出不同的假设。对于研究政策的短期影响时，我们可以假设价格不会变动很大。我们甚至可以做极端人为的假设——所有价格都完全不变。然而，对于研究政策的长期影响，我们可以假设所有价格都是完全弹性的。就像物理学家研究大理石降落和沙滩排球降落用不同的假设一样，经济学家在研究货币量变化的短期和长期影响时，用不同的假设。

High school biology teachers teach basic anatomy with plastic replicas of the human body. These models have all the major organs—the heart, the liver, the kidneys, and so on. The models allow teachers to show their students in a simple way how the important parts of the body fit together. Of course, these plastic models are not actual human bodies, and no one would mistake the model for a real person. These models are stylized, and they omit many details. Yet despite this lack of realism—indeed, because of this lack of realism—studying these models is useful for learning how the human body works.

高中生物老师用人体的塑料复制品讲授基础解剖学。这些模型都有主要的器官：心脏、肝脏、肾脏等等。这些模型让老师用简单的方式展示给他们的学生身体上这些重要的部分是如何互相协调的。当然，这些塑料模型不是真实的人体，没有人会把这些模型误认为是真的人。这些模型是设计的，他们忽略了很多细节。然而，尽管缺乏真实性，但是正是对这些模型的研究，对学习人体如何工作很有帮助。

Economists also use models to learn about the world, but instead of being made of plastic, they are most often composed of diagrams and equations. Like a biology teacher’s plastic model, economic models omit many details to allow us to see what is truly important. Just as the biology teacher’s model does not include all of the body’s muscles and capillaries, an economist’s model does not include every feature of the economy. All models—in physics, biology, or economics— simplify reality in order to improve our understanding of it.

经济学家也用模型来学习世界，但是他们通常用图表和等式，而不是制作塑料复制品。就像一个生物老师的塑料模型，经济模型忽略了很多细节，这些细节可以让我们看到什么是真正重要的。就像生物老师的模型不包括人体所有的肌肉和毛细血管一样，经济学家的模型不包括经济的每个特征。所有的模型不论是物理、生物、还是经济学，都简化了现实，以提高我们对问题的理解。

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