that when the mind proceeded to recount in its own way the suggestions of a perception, it followed a certain logical order with greater or less error according to the quality of mind. Locke assumed that when one idea suggests another to which it is related in a logical way, no further explanation of the association is necessary; but where such association exists between ideas having no logical connection, he falls back on the explanation that they have occurred together accidentally in the past, and because of this we have formed the habit of thinking them together, so that when one recurs we immediately call up the others. If the idea fire suggests that of cooking food, association is not needed to explain the relation; if, instead, London is thought of, we wonder and can account for the fact only by supposing that the experience or description of a chance fire in London has proved so influential as to cause the idea fire to suggest its habitual associate London, rather than its logical or scientific premises or consequence.
Hartley (1704-1757), an English psychologist, uses the idea of association by habit to explain not merely the curious and arbitrary associations of ideas but the logical ones as well. All mental processes are, according to him, dependent on the processes in the brain. Those brain processes that occur together become associated by habit so that the re-excitement of one will cause the re-excitement of the others, and the corresponding ideas will come successively into consciousness. The discoveries of modern experimental psychology make it possible to state still more definitely the character of the associations in the brain. It is known that visual consciousness is dependent upon the excitement of structures in the rear of the brain, and that auditory areas lie in the temporal regions. Suppose that, at the time when one sees a cat, he hears the sound of the word cat Then the auditory and the visual structures are57 excited together. They become so associated that when again we hear the word the nervous currents discharge from the temporal into the occipital or rear portion of the brain, and the visual image of a cat rises in the mind. Apparently the sound of the name suggests immediately the visual image of the thing. Really it is the association of the two portions of the brain corresponding to the two ideas that causes the one to follow the other.
After the time of Hartley," English psychologists attempted to explain all trains of thought on the basis of association. But ^the knowledge of the anatomy and physiology of the brain was too imperfect to lead thinkers to value highly the theory that all mental associations depend upon habitual associations of brain processes. Instead they interest themselves in a statement of the logical relationships existing between ideas that suggest one another. They find that when ideas are similar or contrasting or come in close succession in time, or when their objects are related as cause and effect or are situated near each other in space, they are likely to call each other into consciousness. Professor James, the American psychologist, points out the fact that such laws of association may be increased indefinitely, and that they do not help one to do anything in the way of predicting the effect of a suggestion on a given mind. In his opinion, and here modern psychology agrees substantially with him, the physiological law of habit is the only real explanatory law of association. When we think logically and when our fancy riots in nonsense, we are following the same fundamental law of connecting those experiences that have occurred together in time.
Professor James, however, finds it necessary to expand his treatment of this law. The same idea may have many habitual associates. How are we to tell which one it will suggest? Other things being equal, the most frequent associate will come up. The idea of fire will suggest cooking rather than London, because the former has been associated with it much more commonly. If, however, the experience of the London fire was very exciting, as it might well have been in case the fire occurred at a hotel where we were staying, the association of fire with London might be so intense as to overcome the effect of the frequent association with cooking. Again, if one had just been absorbed in the accounts of the San Francisco disaster the idea of fire might suggest this most recent associate rather than the most frequent or the most intense ones. To know the effect of these principles so that we can predict the result of a given suggestion on a certain mind, it is necessary to know the concrete condition of this mind. Here it is that the subject of the association of ideas connects with that of apperception (q. v.). The experience that any new perception or idea calls up and through which it becomes apperceived depends on the laws of association, and the teacher in counting on the force of any suggestion must know the experience of the pupil and apply these laws. No amount of acquaintance with the logical relationships in the material of instruction will help, unless the teacher knows in how far these logical associations have become habitual in the mind of the pupil.
One further condition affecting the working of the law of association needs to be mentioned. The current of one's thought
