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.

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."

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.

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.

Thus, in hay fever or "rose cold" the tears, the inflammation of the membranes of the nose, the cough, the other trying symptoms, all are linked with the sight of a rose, or dust, or sunlight, or some other outside fact to which attention has been called as the cause of hay fever, into a complex, "an automatically working mechanism." And the validity of this explanation of the regular recurrence of attacks of this disease is sufficiently demonstrated by the fact that a paper rose is likely to prove just as effective in producing all the symptoms of the disease as a rose out of Nature's garden.

Another striking illustration of the working of this principle is afforded by two gentlemen of my acquaintance, brothers, each of whom since boyhood has had unfailing attacks of sneezing upon first arising in the morning. No sooner is one of these men awake and seated upon the edge of his bed for dressing than he begins to sneeze, and he continues to sneeze for fifteen or twenty minutes thereafter, although he has no "cold" and never sneezes at any-other time.

Obviously, if absolutely all mental experiences are preserved, they consist altogether of two broad classes of complexes: first, those that are momentarily active in consciousness, forming part of the present mental picture, and, second, all the others - that is to say, all past experiences that are not at the present moment before the mind's eye.

There are, then, conscious complexes and subconscious complexes, complexes of consciousness and complexes of subconsciousness.

And of the complexes of subconsciousness, some are far more readily recalled than others. Some are forever popping into one's thoughts, while others can be brought to the light of consciousness only by some unusual and deep-probing stimulus. And the human mind is a vast storehouse of complexes, far the greater part buried in subconsciousness, yet somehow, like impressions on the wax cylinder of a phonograph, preserved with life-like truth and clearness.

Turn back for a moment to our definition of memory. You will observe that its second essential element is Recall.

Recall is the process by which the experiences of the past are summoned from the reservoir of the subconscious into the light of present consciousness. We necessarily touched upon this process in a previous book, in considering the Laws of Association, but here, in relation to memory, we shall go into the matter somewhat more analytically.