Though still frequently spoken of as concepts, they are definitely referred to the receptive, non-spontaneous, side of the mind.
1650s, from Late Latin spontaneus "willing, of one's free will," from Latin (sua) sponte "of one's own accord, willingly;" of unknown origin. Related: Spontaneously. Earliest use is of persons and characters. Spontaneous combustion first attested 1795. Spontaneous generation (the phrase, not the event) attested from 1650s.