Full Idea
Whenever the potential active and the potentially affected items are associated in conditions propitious to the potentiality, the former must of necessity act and the latter must of necessity be affected.
Gist of Idea
When a power and its object meet in the right conditions, an action necessarily follows
Source
Aristotle (Metaphysics [c.324 BCE], 1048a08)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.264
A Reaction
Of course the world could end between the two happenings, so this can't be full-scale metaphysical necessity. That point is not enough, though, to get rid of Aristotle's intuition here.