Full Idea
A thing really is if it has any capacity, either by nature to do something to something else or to have even the smallest thing done to it by the most trivial thing, even if it only happens once. I'll define those which are as nothing other than capacity.
Gist of Idea
To be is to have a capacity, to act on other things, or to receive actions
Source
Plato (The Sophist [c.358 BCE], 247e)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.269
A Reaction
If philosophy is footnotes to Plato, this should be the foundational remark in all discussions of existence (though Parmenides might claim priority). It seems to say 'to be is to have a causal role (active or passive)'. It also seems essentialist.
Related Idea
Idea 2329 Causal power is a good way of distinguishing the real from the unreal [Kim]