Full Idea
Demonstration is an argument which by means of things more clearly grasped concludes to something that is less clearly grasped.
Gist of Idea
Demonstration derives what is less clear from what is clear
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.45
A Reaction
In Aristotle demonstration seems to concern physical sciences, but this stoic account makes it sound like pure logic proof. This is why all logic tends to start from atomic sentences, because they are clearest.
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.11