The complex, as defined by C. G. Jung, is a structure of the psyche that gathers together similar feeling-toned elements. Each complex is united by the same emotion and each complex is united and organised by a mutual core of meaning. That is, the complex organises experience, perception, and affect around a constant central theme.
In what way or how is a complex visible? One way is to observe stereotypical behaviour patterns. Of course mankind has made observations of behaviour patterns for countless generations, and we may credit Jung for his methodology on this matter, which was broad in scope. He considered man's reflections on behaviour patterns in other eras and from other cultures. It can be helpful for our understanding of Jung's theory of the complex to look first at the direct forerunners to Jung's theory, and then to go into Jung's use of mythology.
Forerunners who formulated ideas on what Jung will later call the complex include Jean-Martin Charcot, Pierre Janet and Josef Breuer who worked closely with Freud. Each has made insightful observations.
Jean-Martin Charcot, known for his studies on hysterical paralysis, stated in a lecture of May 1885, “[A]n idea, a coherent group of associated ideas settle themselves in the mind in the fashion of parasites, remaining isolated from the rest of the mind and expressing themselves outwardly through corresponding motor phenomena…” (Ellenberger 1970, 149).
Pierre Janet formulated his observations of hysterical patients in terms of fixed ideas. These fixed ideas in hysterical patients are thoughts or mental images that take on exaggerated proportions, have a high emotional charge and become isolated from the habitual personality, or personal consciousness (see Monahan 2009, 40). A fixed idea facilitated the mechanism known in Janet's day assplitting of consciousness' or dissociation.
Jung concurred with Pierre Janet's basic argument that when a person experiences emotions that overwhelm his capacity to take appropriate action the memory of this traumatic experience is split off, and there is dissociation. Janet's view of dissociation was that it was inherently pathological. Jung, in contrast to Janet, saw the dissociability of the psyche as a fundamentally normal and universal phenomenon. Jung was aware of the potentially disastrous psychological consequences of extreme dissociation, but saw dissociation as fundamental to the operation of the psyche. In this view Jung anticipates the modern study of dissociative disorders by envisaging dissociability as a continuum extending from normal to seriously abnormal states. To quote Jung on this matter:
Let us turn first to the question of the psyche's tendency to split. Although this peculiarity is most clearly observable in psychopathology, fundamentally it is a normal phenomenonIt need not be a question of hysterical multiple personality, or schizophrenic alterations of personality, but merely of so-calledcomplexes' that come entirely within the scope of the normal (CW 8, § 253).
The term complex as used by Jung has been generally viewed by the psychoanalytic community as introduced by the Zurich school. However, Josef Breuer used the term complex in this sense already in 1893 in his chapterTheorecticalin the work he co-authored with Freud, “Studies on Hysteria” (see Myers 2009, 516).

In this chapter Breuer comments on Janet's idée fixe writing, “[S]ense-impressions that are not apperceived and the ideas that are aroused but do not enter consciousness….[s]ometimes accumulate and form complexesmental strata withdrawn from consciousness; they form a subconsciousness” (Ibid.).
What these famed observers are noting is that a traumatic event can overwhelm the ego and bring about dissociation of the psyche.