Descartes lived at a time when the established wisdom was Aristotle's idea of the soul, and Aristotle's soul, after all, encompassed the whole body. It was the all-pervading activating feature of all body functions, of all life.1,15,25,26 This was upheld even as late as the 17th century by men of the caliber of Harvey, Glisson, and van Helmont. Modern physiology—physical, chemical, molecular physiology—would have been impossible under it. Descartes at least takes the soul out of the heart (where Aristotle at one time had it), the blood (where Harvey had it), the pyloric end of the stomach (where van Helmont had it), and even out of the brain itself. As an immaterial entity, the soul is not strictly localizable, but interacts with the body—and this interaction is limited to one small gland. This allows the body itself to be understood in material, mechanistic terms and overcomes a great philosophical hurdle. From this point on, Aristotle's soul began to wither away. “There is a mask of theory over the whole face of nature,” said the 19th century philosopher of science William Whewell.27 Now the mask was changing. The terms of the problem were being redefined.