Entry written 4 November 2005:
Quoting C.G. Jung--

To the reader who knows little or nothing of my work, a word of explanation may be helpful. Some thirty-five years ago I noticed to my amazement that European and American men and women coming to me for psychological advice were producing in their dreams and fantasies symbols similar to, and often identical with, the symbols found in the mystery religions of antiquity, in mythology, folklore, fairytales, and the apparently meaningless formulations of such esoteric cults as alchemy. Experience showed, moreover, that these symbols brought with them new energy and new life to the people to whom they came.

From long and careful comparison and analysis of these products of the unconscious I was led to postulate a 'collective unconscious,' a source of energy and insight into the depth of the human psyche which has operated in and through man from the earliest periods of which we have records.

In this present study of alchemy I have taken a particular example of symbol-formation, extending in all over some seventeen centuries, and have subjected it to intensive examination, linking it at the same time with an actual series of dreams recorded by a modern European not under my direct supervision and having no knowledge of what the symbols appearing in the dreams might mean. It is by such intensive comparisons as this (and not one but many) that the hypothesis of the collective unconscious--of an activity
in the human psyche making for the spiritual development of the individual human being--may be scientifically established.

from Psychology and Alchemy, "Prefatory Note to the English Edition," by C.G. Jung (Bollingen Series XX, vol.12, 2nd edition, Princeton Univ. Press, 1968)