QUOTE : By exerting its will, Descartes declared, the immaterial human mind could cause the material human machine to move. This bears repeating, for it is an idea that, more than any other, has thrown a stumbling block across the path of philosophers who have attempted to argue that the mind is immaterial: for how could something immaterial act efficaciously on something as fully tangible as a body? Immaterial mental substance is so ontologically different-that is, such a different sort of thing-from the body it affects that getting the twain to meet has been exceedingly difficult. To be sure, Descartes tried. He argued that the mental substance of the mind interacts with the matter of the brain through the pineal gland, the organ he believed was moved directly by the human soul. The interaction allowed the material brain to be physically directed by the immaterial mind through what Descartes called "animal spirits"-basically a kind of hydraulic fluid.