Full Idea
Necessity is what makes it impossible for something to be other than it is.
Gist of Idea
Necessity makes alternatives impossible
Source
Aristotle (Metaphysics [c.324 BCE], 1015b03)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.120
A Reaction
Note that necessity here seems like an active force, rather than a mere description of a logical or metaphysical state of affairs. The underlying idea seems to be that essences enforce necessities, but it doesn't say that here.