And in fact, considering the complexity of mental life, it would seem probable that simple associatlons, by similarity or contiguity alone, never occur. Besides these primary laws of association, various secondary laws are enumerated, such as the laws of frequency, vividness, recentness, emotional congruity, etc. These determine the firmness of the association, and consequently the preference given to one state over another, in the recall. Association of ideas is a fact of everyday experience which furnishes an important basis for the science of psychology, yet it must be remembered that the laws of association offer no ultimate explanation of the facts observed. In accounting for the facts of association we must, in the first place, reject as insufficient the purely physical theory proposed by Ribot, Richet, Maudsley, Carpenter, and others, who seek an explanation exclusively in the association of brain processes.
Psychology thus becomes a chapter of physiology and machanics.
Aside from the fact that this theory can give no satisfactory explanation of association by similarity which implies a distinctly mental factor, it neglects evident facts of consciousness. Consciousness tells us that in reminiscence we can voluntarily direct the sequence of our mental states, and it is in this that voluntary recall differs from the succession of images and feelings in dream and delirium. Besides, one brain-process may excite another, but this is not yet a state of consciousness. Equally unsatisfactory is the theory of the ultra-spiritualists, who would have us believe that association ot ideas has nothing to do with the bodily organism, but is wholly mental. Thus Hamilton says that all physiological theories are too contemptible for serious criticism.