In chapter 23 of The Contradictions of the Stoics, Plutarch, far from referring to the Epicurean
theory of clinamen, gives a puzzling description of the academic interpretation of the theory
of ἐπελεύσεις/occurrentia by Ariston of Chios. Arguing against the doctrine of the Stoic
Ariston, Arcesilaus tried to demonstrate to his antagonist that the assent does not serve as
a starting point for the impulse when one is forced to opt for one of two indistinguishable
representations (e. g. to choose one of two identical drachmas). In order to release the impulse
from the tyranny of such representations, and not to fall into paralysis, some philosophers
– according to our hypothesis, Arcesilaus and Academics in controversy with the Stoic
Ariston –, have located in the central commanding faculty of the soul a kind of adventitious
movement or faculty that, providing an instantaneous inclination, sets in motion the impulse
without the need of assent.