Full Idea
If the argument isn't that, metaphysically speaking, anything that exists must have causal powers - how on earth would we show that? - rather, the claim is an epistemic one. Any thing we're in a position to know about we must causally interact with.
Gist of Idea
That all existents have causal powers is unknowable; the claim is simply an epistemic one
Source
Jody Azzouni (Deflating Existential Consequence [2004], Ch.4)
Book Reference
Azzouni,Jody: 'Deflating Existential Consequence' [OUP 2004], p.83
A Reaction
A very good point. I am attracted to causal power as a criterion for existence, but Azzouni's distinction is vital. Maybe there is just no point in even talking about things which exist but have no causal powers.