Collected Works of C.G. Jung, Volume 9 (Part 1): Archetypes and the Collective Unconscious
We must now turn to the question of how the existence of archetypes can be proved. Since archetypes are supposed to produce certain psychic forms, we must discuss how and where one can get hold of the material demonstrating these forms.
The main source, then, is dreams, which have the advantage of being involuntary, spontaneous products of nature not falsified by any conscious purpose. By questioning the individual one can ascertain which of the motifs appearing in the dream are known to him…
Consequently, we must look for motifs which could not possibly be known to the dreamer and yet behave functionally of the archetype known from historical sources. ~Carl Jung; The Archetypes and the Collective Unconscious; Page 48.