In 1644, Descartes formulated one of the basic elements of all modern science. In his Principes de la philosophie, he fixed what once for all would be the dividing line between theology and science. With Descartes' analysis in mind, one would never have to mix the concepts, lest one produce bad science.

Before that, Galileo pointed the way to mathematics as the only language with sufficient clarity and precision to be able to handle all of nature's phenomena. Galileo had, in addition, demonstrated the importance of humbly and sensitively questioning nature itself through rigorous and empirical experiments in which dreams and preconceived notions are replaced by indisputable facts. But the conclusion Descartes came to was, in many ways, even more profound and revolutionary, which was that scientists must never look for reasons based on future purposes. No explanation of natural phenomena should be based in the existence of a p...