Full Idea
All circles of interdefinition are useless by themselves. But if we reach one of the interdefined pair, then we have them both.
Gist of Idea
Interdefinition is useless by itself, but if we grasp one separately, we have them both
Source
David Lewis (Defining 'Intrinsic' (with Rae Langton) [1998], IV)
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.121