Full Idea
Stoics define knowledge as an assertion or safe comprehension or habit, which, in the perception of what is seen, never deviates from the truth.
Clarification
'Knowledge' here is the Greek word 'epistemé'
Gist of Idea
For Stoics knowledge is an assertion which never deviates from the truth
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.1.25
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.276
A Reaction
Sounds somewhere between Nozick's 'tracking the truth' and Goldman's 'reliable source'. If the world is a flux, then presumably it is right that knowledge should fluctuate too.