Full Idea
Actual events are evidently possible, otherwise they would not have occurred.
Gist of Idea
The actual must be possible, because it occurred
Source
Aristotle (The Poetics [c.347 BCE], 1451b18)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.1464
A Reaction
[quoted online by Peter Adamson] Seems like common sense, but it's important to have Aristotle assert it.