Influenced from mercantilism and physiocracy theories, it toke place from the late XVIII century to the late XIX century. It is considered that its main authors were Adam Smith, David Ricardo and John Stuart Mill, and the fourth, the unorthodox Robert Malthus. They wrote specially about the theory of value, distribution theory and international trade. Karl Marx studied the same matters, although with different conclusions and defending the working class, which makes him a classical economist in the eyes of some historians. Thanks to these authors, the study of economics became more of a science, instead of just being some kind of philosophy.