spontaneity (n.)

1650s, "occurring without external stimulus," from Late Latin spontaneus "willing, of one's free will," from Latin (sua) sponte "of one's own accord, willingly," a word of uncertain origin. Related: Spontaneously; spontaneousness. Used earlier of persons and characters, with a sense "acting of one's own accord" (c. 1200). Spontaneous combustion first attested 1795. Spontaneous generation (the phrase, not the feat) attested from 1650s.

word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).

Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]