Full Idea
If we have a single counter-instance, the argument is refuted as not necessary, even if more cases are otherwise or more often otherwise.
Gist of Idea
A single counterexample is enough to prove that a truth is not necessary
Source
Aristotle (The Art of Rhetoric [c.350 BCE], 1403a07)
Book Reference
Aristotle: 'The Art of Rhetoric', ed/tr. Lawson-Tancred,H.C. [Penguin 1991], p.213
A Reaction
This is Aristotle (pioneering hero) pointing out what we now tend to think of as Karl Popper's falsification, the certain way to demonstrate the falseness of a supposed law of nature, by finding one anomaly from it.