Full Idea
If anything has the property of being perishable it has it of necessity, on pain of one and the same thing being perishable and imperishable.
Gist of Idea
A thing has a feature necessarily if its denial brings a contradiction
Source
Aristotle (Metaphysics [c.324 BCE], 1059a05)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.314
A Reaction
Of course the perishable could become imperishable over time, without contradiction. This illustrates the foundational idea that a proposition is necessary if its negation is a contradiction. [...actually this argument is invalid as it stands!]