David Ricardo's introduction to economic theory was by chance. It is said that he discovered Adam Smith's The Wealth of Nations during a retreat, in his late twenties. Reading this book made an impact on him, and, like many others in those times, turned him into a convinced follower of Smith's way of thinking. Ricardo was a self-taught economist, and he was not that young when he actually became one. Be it as it may, he adhered to Smith's main work, and, from then on, at the same time he took care of his commercial activities, he dedicated himself to writing articles on economics for the British press. Many years later, when he had finally retired from his profession, his friends encouraged him to write a book in which he could formalize his ideas regarding economics. As a consequence, his great work, On the Principles of Political Economy and Taxation, was born. Despite this book is relatively short, it is somewhat complex. Furthermore, coherent formulation of classical Ricardo, this movement included Robert Malthus, and John Stuart it is considered to contain the most systematic and economic thought. Besides Adam Smith and David other thinkers such as Jean-Baptiste Say, Thomas Mill.