The detachment of libido from the object [its concentration on the symbol] transfers it into the subject, where it activates the images lying dormant in the unconscious. These images are archaic forms of expression which become symbols.
Source - C.G. Jung, Carl Spitteler: Prometheus and Epimetheus (The Reality of the Symbol). CW 6: Psychological Types, par. 402.
Symbols want to be mysterious; they are not so merely because what is at the bottom of them cannot be clearly apprehended.
Source - C.G. Jung, Letter to Hans Schmid, 6 November 1915. C.G. Jung Letters 1906-1950 (Routledge 2015), Vol. 1, p. 31.
The symbol has a very complex meaning because it defies reason; it always presupposes a lot of meanings that can't be comprehended in a single logical concept. The symbol has a future. The past does not suffice to interpret it, because germs of the future are included in every actual situation. That's why, in elucidating a case, the symbolism is spontaneously applicable, for it contains the future; within its zone of mystery, it comprises the individual's defense. For example, a developing disease always has a counter-aspect: together with fever as a germ infection, there is simultaneously fever as a bodily reaction and defense.
Source - C.G. Jung, C.G. Jung Speaking: Interviews and Encounters (Princeton University Press 1977), p. 143.
Art: Raphaël Vavasseur