GDP, which only measures the value of the final product, ignores the various production processes that take place in the real world before the final product is produced. In this sense, GDP is not a reflection of the real world, but rather an artificially constructed measure of a ‘fictitious economy’. A bigger problem with GDP is that the identity equation, by which GDP is expressed as the sum of consumer spending, business investment spending, government spending, and net exports, is used in a causal way. Using this identity equation, governments often implement policies that increase spending to stimulate the economy and grow the economy. However, the principle of human behavior that one must first produce in order to consume implies that production is the cause and consumption is the effect. The idea, which implies that consumption is the cause, is thus incorrect because it goes against the principle of human behavior. This idea is based on Keynes's theory. Keynes's theory, as presented in his General Theory, is a misinterpretation of Say's Law of the Market, which states that “when one product is produced, it creates a demand for another product,” as “supply creates its own demand.” Macroeconomics, which was started based on the wrong Keynes' theory, is also a theory that is far from the real world and does not explain the economic problems that are occurring in the real world, such as the financial crisis. Therefore, in order for macroeconomics to properly explain the real world, it is necessary to establish a new macroeconomics. The new macroeconomics should be based on the principles of human behavior and should include the actual production process and the activities of entrepreneurs, who play an important role in the production process. The Austrian Business Cycle Theory, which covers these aspects, can serve as the cornerstone of a new macroeconomic theory.