Jean-Baptiste Say (1767-1832) was one of the most central economists in the development of classical economics at the start of the 19th century, and the discoverer of among others Say’s law. This law is one of the essential laws of economics, and its validity and scope has received much scholarship. But the same cannot be said about Say’s methodology, i.e. how he discovered this and other laws of economics. Here the previous literature (Reynaud, Forget, and Rothbard) is far from satisfactory. I argue that Say both in his methodological introduction and throughout the main body of his A Treatise on Political Economy, applied the excellent methodology of Newton (e.g. his 4 rules of reasoning) to the science of economics. Recent literature (Redman, Montes, and Schliesser) has defended a similar thesis on Adam Smith, but Smith wrote remarkably little on methodology. In addition, despite of Say naming him the founder of economics, Smith is more a transitionary figure while Say is in fact the paradigm case of a classical economist.