Jean-Baptiste Say Popularized the Arguments of Adam Smith in France, Introduced a Fourth Factor of Production: The Entrepreneur, and Popularized an Economic Principle Now Called Say's Law

Given his subsequent importance, the French economist Jean-Baptiste Say must be brought into the story. Say was for the most part a popularizer of the arguments that had been presented by Adam Smith. His Treatise on Political Economy was published in 1803 to acquaint the French public with Smith's ideas. But it was not merely a restatement of Smith: Say had a number of innovations of his own.

The first of these was the introduction of a fourth factor of production beyond land, labour and capital. This was the additional input that was brought to the production process by the entrepreneur. Say explicitly recognized the crucial importance of the entrepreneur as the initiator and organizer of the production process. It was, as he wrote, through the entrepreneur that value-adding activity was able to take place.

For reasons unknown, independent discussion of the role of the entrepreneur remains relatively uncommon even to this day. Yet without the entrepreneur to identify value-adding possibilities, introduce innovation and then superintend the process all along the way, value-adding production would remain far less common and prosperity would possibly have remained as elusive as it had been in all of the centuries prior to the arrival of the industrial revolution.

But Say's other innovation was the popularization of an economic principle which would be given the name Say's Law, but not until more than a century had gone by since its first discussion by Say in his Treatise. 

Although there were a number of strands to the surrounding principle, in brief it may be stated as: demand for goods and services is created by value-adding production and by nothing else. For goods to be bought not only must goods be produced, but precisely those goods that others would be willing to buy must be the ones that need to be produced.

--Steven Kates, Free Market Economics: An Introduction for the General Reader, 3rd ed. (Cheltenham, UK: Edward Elgar Publishing, 2017), Kobo e-book.