Full Idea
To say that everything that is, is of necessity, when it is, is not the same as saying unconditionally that it is of necessity.
Gist of Idea
Things may be necessary once they occur, but not be unconditionally necessary
Source
Aristotle (On Interpretation [c.330 BCE], 19a25)
Book Reference
Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.53