Full Idea
That proposition is possible which admits of being true, if external factors do not prevent it from being true, for example, 'Diocles is alive'.
Gist of Idea
A proposition is possible if it is true when nothing stops it being true
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.75
A Reaction
Well that's different. So every unprevented possibility will occur tomorrow. Any possibility that does not occur tomorrow must have been prevented in some way. Whatever does occur prevents innumerable other things from occurring. Your turn…
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.19