A reflex is what happens when you strike the patellar tendon of the knee with the side of your hand, with a tack-hammer, or with what Dr. Watson prefers to call “a percussion hammer.” The leg registers a kick, because the blow sets up a wave of chemical decomposition in the dendrites of the receiving neurones which are nearest concerned; the wave passes to certain association neurones, and by them is passed to the kicking (striped) muscle. That is, it passes through a reflex arc, one of the several reflex arcs which are already in working order when we are born. Which means that the association neurones here involved have somehow already been educated to relay all despatches to the neurones which operate the kick--and to none other. So this reflex arc is called “unconditioned,” and the stimulus which sets it in motion is said to be a “natural” stimulus. An unconditioned reflex is practically a tropism. Prof. Loeb ordered it to get busy and account for all behavior, human and other. It fell down on the job. We have not, it seems, enough knee and similar jerks to enable us to write a Ninth Symphony with their aid alone. Some other trifle was needed. Dr. Watson says he has it--in the conditioned reflex. It is possible, we find, to get a reflex to respond to a stimulus which is not its “natural” stimulus. Food, for example, is the natural stimulus of the reflex arc operating the salivary glands. The mouth waters when we begin to eat. But the Russian, Pavlov, bored holes in the cheeks of a dog, brought the salivary ducts into the open, and discovered that not only would the dog’s mouth water at the contact of food, but that it would water at the mere sight of food. Then he rang a bell for several days at feeding time, and had the satisfaction eventually of seeing the dog’s mouth begin to water when the bell was rung and no food was anywhere about. Prof. Lashley, of the Hopkins Laboratory, has since invented an instrument which can be fastened to the inside of the cheek by means of suction, so that the secretions may be led to a tube without a surgical operation--which is a convenience when the victim is human. He has thus ascertained how many drops per minute was the normal slaver, and that chocolate in the (human) subject’s hand increased it from three to five times; the smell of chocolate, five times; chocolate brought to the lips, nine times. “It may be said,” observes Bertrand Russell[6] “that modern psychology consists of the discovery by the professors of what everybody else has always known.” Yet it must be confessed that we did not know the number of drops of saliva which lay behind the licking of our chops in the presence of something good, or that the sight and smell of chocolate were “unnatural” or “substituted” stimuli--still less that the substitution of unnatural for natural stimuli was both the foundation and superstructure of “education.”