Although I do not agree with Dr. Watson in thinking this principle alone sufficient, I do agree that it is a principle of very great importance. It is the modern form of the principle of “association”. The “association of ideas” has played a great part in philosophy, particularly in British philosophy. But it now appears that this is a consequence of a wider and more primitive principle, namely, the association of bodily processes. It is this wider principle that is asserted above. Let us see what is the nature of the evidence in its favour. Our principle becomes verifiable over a much larger field than the older principle owing to the fact that it is movements, not “ideas”, that are to be associated. Where animals are concerned, ideas are hypothetical, but movements can be observed; even with men, many movements are involuntary and unconscious. Yet animal movements and unconscious involuntary human movements are just as much subject to the law of association as the most conscious ideas. Take, _e.g._ the following example (Watson, p. 33). The pupil of the eye expands in darkness and contracts in bright light; this is an involuntary and unconscious action of which we only become aware by observing others. Now take some person and repeatedly expose him to bright light at the same moment that you ring an electric bell. After a time the electric bell alone will cause his pupils to contract. As far as can be discovered, all muscles behave in this way. So do glands where they can be tested. It is said that a brass band can be reduced to silence by sucking a lemon in front of it, owing to the effect upon the salivary glands of its members; I confess that I have never verified this statement. But you will find the exact scientific analogue for dogs in Watson, p. 26. You arrange a tube in a dog’s mouth so that saliva drops out at a measurable rate. When you give the dog food it stimulates the flow of saliva. At the same moment you touch his left thigh. After a certain length of time the touch on the left thigh will produce just as much saliva without the food as with it. The same sort of thing applies to emotions, which depend upon the ductless glands. Children at birth are afraid of loud noises, but not of animals. Watson took a child eleven months old, who was fond of a certain white rat; twice at the moment when the child touched the rat, a sudden noise was made just behind the child’s head. This was enough to cause fear of the rat on subsequent occasions, no doubt owing to the fact that the adrenal gland was now stimulated by the substitute stimulus, just like the salivary glands in the dog or the trumpet player. The above illustrations show that “ideas” are not the essential units in association. It seems that not merely is “mind” irrelevant, but even the brain is less important than was formerly supposed. At any rate, what is known experimentally is that the glands and muscles (both striped and unstriped) of the