Full Idea
What had been grasped by sense-perception, he called this itself a 'sense-perception', and if it was grasped in such a way that it could not be shaken by argument he called it 'knowledge'. And between knowledge and ignorance he placed the 'grasp'.
Gist of Idea
If a grasped perception cannot be shaken by argument, it is 'knowledge'
Source
report of Zeno (Citium) (fragments/reports [c.294 BCE]) by M. Tullius Cicero - Academica I.41
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.40
A Reaction
This seems to say that a grasped perception is knowledge if there is no defeater.