Say’s chief work “Traité d’économie politique”, 1803, gathered up many of the ideas of his French predecessors and Adam Smiths, systematizing this masterful treatise, which nevertheless includes many original contributions. Thanks to it he gained recognition in Europe and at the other side of the Atlantic.
However, Say’s most important contribution to Economics was his apparently simple formulation of the Say’s Law (1803) that states, “products are paid for with products”. Later, in 1808, Scottish economist James Mill (father of the renowned John Stuart Mill), rephrased it as “production of commodities creates […] a market for the commodities produced”, and J. M. Keynes would rephrase it (although whilst making a point against it) in his “General Theory”, 1936, as “supply creates its own demand”, which is the better known restatement.
Jean-Baptiste Say was in touch with many economists of his era and carried out moderate debates with them, he distinguished himself as a more policy-oriented economist rather than a model-builder like David Ricardo.