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Full Idea
The possibility of action ...is the criterion of existence, and the test of substantiality.
Gist of Idea
The criterion of existence is the possibility of action
Source
George Santayana (The Realm of Matter [1930], p.107), quoted by John Heil - The Universe as We Find It
Book Ref
Heil,John: 'The Universe as We Find It' [OUP 2012], p.53
A Reaction
I rather like this. I think I would say the power is the criterion of existence.