In the early 1600s, long before there were airplanes, telephones or computers, an English doctor was performing some curious experiments with snakes, fish, and later humans. The doctor would tie their veins and arteries in various places, sometimes very tightly, other times less so, and he would closely observed the effects on the flow of blood to and from the heart. What William Harvey had discovered is that the heart is a pump which circulates the blood throughout the body. This was a major step forward for the science and practice of medicine. But it also had a larger meaning. The implications of which Harvey and his contemporaries could scarcely imagine. One of those contemporaries was the French philosopher, Rene Descartes. A few years after Harvey's discovery, Descartes would undertake his famous meditations. He closed himself in a room, sitting quietly and meditating. This is how he described it, "I shall now close my eyes, I shall stop my ears, I shall call away all my senses, I shall efface even from my thoughts, all the images of corporeal things, or at least, for that is hardly possible, I shall esteem them as vain and false, and thus holding converse only with myself and considering my own nature, I shall try little by little to reach a better knowledge of and a more familiar acquaintanceship with myself." Descartes' meditation we're incredibly wide ranging and included momentous topics such as the existence of God, the existence of the soul, and the very foundations of human knowledge. A century before the French and American Revolutions, Descartes had produced a declaration of independence for the human mind. But there's one topic that seems quite at a place. Descartes goes on for page after page describing Dr. Harvey's discovery about the way the heart pumps blood throughout the human body. Why should this seemingly mundane issue find a place in Descartes meditations on the most profound topics of philosophy? Harvey's discovery forces us to confront some uncomfortable questions, questions that have only become more uncomfortable in the subsequent centuries. If the heart is a pump, then isn't the rest of the human body also a machine? Could you build such a machine? Specifically, could you build a machine that duplicates the body of a human or perhaps a monkey? If you did build such a machine, could you tell the difference between the machine and the real living being? As far as monkeys go, Descartes' answer was clear, you could indeed build a monkey machine. There's nothing a monkey can do that can't be duplicated by a machine. What about humans? Here, Descartes was equally clear. It is not possible. If you try to build a human machine, we will always be able to tell the difference between the machine and a real human. As he put it, "If you built such machines, there would be very certain tests by which to recognize that they weren't real men." What were these very certain tests Descartes had in mind? He explained, "We can easily understand the machine's being constituted so that it can utter words, and even emit some responses to action on it. But it never happens that it arranges its speech in various ways in order to reply appropriately to everything that may be said in its presence, as even the lowest type of man can do." What Descartes was talking about, is the human ability to engage in conversation, a completely ordinary ability, which, as he said, even the lowest type of man can do. Why was Descartes so confident that no machine could duplicate this ability? There is no simple recipe for conversation. It's completely open-ended. You can't predict what your conversational partner will say next. As Descartes said, "Human intelligence is a universal instrument which can serve for all contingencies, while machines have need of some special adaptation for every particular action." Descartes was saying that when we design and build a machine, we create a repertoire of capabilities for the machine. The machine can't come up with something new on its own. But that's what's needed for ordinary conversation. What Descartes was suggesting is that there's an unbridgeable gulf between the open-ended flexibility of human intelligence and the abilities of machines where all of their actions have to be predetermined. For Descartes, there was one crucial thing that machines can't do, they can't think.