[Illustration: Decorative Header] CHAPTER III THE MECHANISM OF RECALL [Sidenote: _The Right Stimulus_] Somehow, somewhere, all experiences, whether subject to voluntary recall or not, are preserved, and are capable of reproduction when the right stimulus comes along. And it is a law that _those experiences which are associated with each other, whether ideas, emotions or voluntary or involuntary muscular movements, tend to become bound together into groups, and these groups tend to become bound together into systems_. [Sidenote: _"Complexes" of Experience_] Such a system of associated groups of experiences is technically known as a "complex." Pay particular attention to these definitions, as "groups" of ideas and "complexes" of ideas, emotions and muscular movements are terms that we shall constantly employ. You learned in a former lesson that mental experiences may consist not only of sense-perceptions based on excitements arising in the memory nerves, but also of bodily emotions, the "feeling tones" of ideas, and of muscular movements based on stimuli arising in the motor nerves. _Groups consist, therefore, not only of associated ideas, but of associated ideas coupled with their emotional qualities and impulses to muscular movements._ All groups bound together by a mutually related idea constitute a single "complex." Every memory you have is an illustration of such "complexes." [Sidenote: _The Thrill of Recollection_] Suppose, for example, you once gained success in a business deal. Your recollection of the other persons concerned in that transaction, of any one detail in the transaction itself, will be accompanied by the faster heartbeat, the quickened circulation of the blood, the feeling of triumph and elation that attended the original experience. [Sidenote: _"Complexes" and Functional Derangements_] Complexes formed out of harrowing earthquakes, robberies, murders or other dreadful spectacles, which were originally accompanied on the part of the onlooker by trembling, perspiration and palpitation of the heart, when lived over again in memory, are again accompanied by all these bodily activities. Your memory of a hairbreadth escape will bring to your cheek the pallor that marked it when the incident occurred. The formation and existence of "complexes" explains the origin of many functional diseases of the body--that is to say, diseases involving no loss or destruction of tissue, but consisting simply in a failure on the part of some bodily organ to perform its allotted function naturally and effectively. [Sidenote: _Automatically Working Mental Mechanisms_]