I believe that Descartes used a variant of the ancient method of analysis and synthesis in his scientific and philosophical works. Some have argued this general interpretive point before, but there has been little agreement about what this method involves.¹ I hope to make a contribution here. A considerable amount of evidence exists supporting the general thesis. Some of it is indirect, displaying that Descartes sought just the sort of axiomatically structured scientific theories that the ancients used the method of analysis and synthesis to establish. Some of it is more direct, to be found in his explicit descriptions of his method, usually in the philosophical works, and in some striking accounts of his own scientific reasoning.