Full Idea
The Permutation Theorem says that any theory with a non-trivial model has many distinct isomorphic models with the same domain.
Gist of Idea
Permutation Theorem: any theory with a decent model has lots of models
Source
Tim Button (The Limits of Reason [2013], 02.1)
Book Reference
Button,Tim: 'The Limits of Realism' [OUP 2013], p.15
A Reaction
This may be the most significant claim of model theory, since Putnam has erected an argument for anti-realism on it. See the ideas of Tim Button.