Full Idea
A plausible criterion for distinguishing what is real from what is not real is the possession of causal power.
Gist of Idea
Causal power is a good way of distinguishing the real from the unreal
Source
Jaegwon Kim (Mind in a Physical World [1998], §4 p.119)
Book Reference
Kim,Jaegwon: 'Mind in the Physical World' [MIT 2000], p.119
A Reaction
This is, of course, a physicalist view, but for physicalists it is probably the best criterion of what is real. A standard objection to platonism in mathematics is that it denies mathematics causal powers.
Related Idea
Idea 7022 To be is to have a capacity, to act on other things, or to receive actions [Plato]