Full Idea
Chrysippus regarded power to act and be acted upon as the criterion for existence or being - a test satisfied by bodies alone.
Gist of Idea
Chrysippus says action is the criterion for existence, which must be physical
Source
report of Chrysippus (fragments/reports [c.240 BCE]) by Teun L. Tieleman - Chrysippus
Book Reference
'Encyclopedia of Classical Philosophy', ed/tr. Zeyl,Donald J. [Fitzroy Dearborn 1997], p.138
A Reaction
This defines existence in terms of causation. Is he ruling out a priori a particle (say) which exists, but never interacts with anything? If so, he is inclining towards anti-realism.