Book excerpt: The era of modern economics emerged with the publication of Carl Menger?s seminal work, Principles of Economics, in 1871. In this slim book, Menger set forth the correct approach to theoretical research in economics and elaborated some of its immediate implications. In particular, Menger sought to identify the causal laws determining the prices that he observed being paid daily in actual markets.4 His stated goal was to formulate a realistic price theory that would provide an integrated explanation of the formation of market phenomena valid for all times and places.5 Menger?s investigations led him to the discovery that all market prices, wage rates, rents, and interest rates could ultimately be traced back to the choices and actions of consumers striving to satisfy their most important wants by ?economizing? scarce means or ?economic goods.? Thus, for Menger, all prices, rents, wage, and interest rates were the outcome of the value judgments of individual consumers who chose between concrete units of different goods according to their subjective values or ?marginal utilities? to use the term coined by his student Friedrich Wieser. With this insight was born modern economics.