In order to find a substitute for laboratory experiments, economists pay close attention to the natural experiments provided by history. When a war in the Middle East interrupted the flow of crude oil, for instance, oil prices soared up throughout the world. For consumers in oil and oil products, such an event has reduced their living standards. For economic policymakers, it puts forward the problem of choosing the best response. But for economic scientists, it provides an opportunity to study the impact of key natural resources on the world economy, and this opportunity will persist for long after the wartime increase in oil price is over. Therefore, throughout this book we will consider a lot of historical events. These events are valuable to study because they enable us to understand the economy of the past, and more importantly, because they allow us to illustrate and evaluate the current economic theories.