to awaken one another. This is what is commonly described as association of ideas. The reverse is also true, although not as well known; many states tend to prevent the appearance of other states. Agreeable states, for example, tend to force out the disagreeable, and vice versa. This, then, may be generalized in the statement of the second property of mental states: every mental state tends to produce, or prevent the production of, other states. We may suppose that these phenomena are due to the interplay of systems of activities within the cortex with one another.
But these activities within the cortex which underlie our ordinary life of sensation and thought tend also to discharge downward through the Rolandic region into the motor mechanism, producing contractions of the muscles. Thus the third property of the mental state is the ability to produce or prevent muscular contraction. Not all mental states have this property in the same degree. It is most evidently true of the feeling of movement. I once asked a class of sixteen girls to think intently what it would feel like to lift the right hand and touch the left shoulder. After a few minutes had elapsed nine of them confessed having felt a desire to do it. I then dropped the subject and spoke of something else: in a few moments six actually did it. Most persons when concentrating attention upon the thought of what a given movement would feel like, find themselves becoming possessed of a desire to do it, and this desire marks the tendency of the thought to produce the movement. But as we not only feel but also see our movements, we find that the thought of what a movement looks like has also motor value and tends to produce it. This is also true of touches and ideas of touch—indeed, all or nearly all mental states produce some motor changes in the body, but the motor effects of sensations and ideas of sound, taste, and smell are relatively slight.
Again, mental states tend to help or hinder the processes of secretion and nutrition. We all know that the secretions of the salivary glands, of the kidneys, of the mammary, and other glands are readily affected by many mental states, but their effect upon the processes of nutrition is more disputed. It is quite certain that in a general way the impulses sent out by the central nervous system are necessary to the proper nutrition of the body, but it is not as generally accepted that individual mental states can produce definite changes, as, for example, when it is reported that a hypnotized patient, by thinking of a burn, has actually produced the burn. Yet even for this there is good evidence.
One may justly ask how it is, if mental states really have definite consequences, that we fail to note in our mental life the orderly sequence of cause and effect with which we are familiar in the physical world? If there be any truth in the theory above
