Both Uses

spontaneous
in
Preface to Lyrical Ballads
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For all good poetry is the spontaneous overflow of powerful feelings: but though this be true, Poems to which any value can be attached, were never produced on any variety of subjects but by a man, who being possessed of more than usual organic sensibility, had also thought long and deeply.†
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spontaneous = behaving in an instinctive, uninhibited manner OR happening naturally (without planning or external force)
I have said that Poetry is the spontaneous overflow of powerful feelings: it takes its origin from emotion recollected in tranquillity: the emotion is contemplated till by a species of reaction the tranquillity gradually disappears, and an emotion, kindred to that which was before the subject of contemplation, is gradually produced, and does itself actually exist in the mind.†

Definitions:

(1) (spontaneous)
happening naturally, suddenly, or without outside planning or action
(2) (meaning too rare to warrant focus)