Full Idea
A concept is a phantasm of the intellect of a rational animal.
Gist of Idea
Concepts are intellectual phantasms
Source
report of Stoic school (fragments/reports [c.200 BCE]) by Pseudo-Plutarch - On the Doctrine of the Philosophers 900c
A Reaction
No doubt they assume that the brutes are devoid of all concepts, but that makes it hard to explain their behaviour.
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.48