Rene Descartes, a 16th century Frenchman often regarded as one of the founders of modern philosophy, penned one of its most famous slogans: “I think, therefore I am.” This statement was the conclusion to an approach of systematic doubt, the goal of which was to arrive at a new foundation for knowledge. He might have been able to doubt all the evidence of his senses, Descartes reasoned, but he could not doubt the proposition that he himself existed to do the doubting. His statement might almost be rephrased as: “I doubt, therefore I am.” Yet despite the fact that philosophers since the early 19th century have made heavy use of this conclusion, Descartes in his own day was most influential as a scientist, not a philosopher.

Throughout 16th century Europe, for example, Descartes was well known for his work in physics, geometry, and physiology, and modern scholars still consider him one of the precursors of the scientific revolution. His attempts to use algebra to solve problems of space and motion essentially created the field of analytic geometry, upon which modern applied mathematics is still largely based. Descartes discovered the law of refraction, which explains why straight objects appear bent when immersed partly in water, and was one of the first to theorize that the workings of human bodies can be studied in similar fashion to the workings of machines. Even his “Discourse on Method,” which contained the famous philosophical statement, was intended merely as the framework within which a collection of essays on optics, meteorology, and geometry was to be understood.

Yet despite the fact that philosophers since the early 19th century have made heavy use of this conclusion, Descartes in his own day was most influential as a scientist, not a philosopher