The dynamic principle of fantasy is play, a characteristic also of the child, and as such it appears inconsistent with the principle of serious work. But without this playing with fantasy no creative work has ever yet come to birth. The debt we owe to the play of imagination is incalculable. It is therefore short-sighted to treat fantasy, on account of its risky or unacceptable nature, as a thing of little worth.
Ch. 1, p. 82 - Psychological Types, or, The Psychology of Individuation (1921)