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Full Idea
A surface-functional characterisation is not exhaustive. It allows unintended twins, alien intruders with different structures - water lookalikes that are not H2O and lookalike infinite structures that are not the natural numbers.
Gist of Idea
Surface accounts aren't exhaustive as they always allow unintended twin cases
Source
Joseph Almog (Nature Without Essence [2010], 03)
Book Ref
-: 'Journal of Philosophy' [-], p.366
A Reaction
He rests this on the claim in mathematical logic that fully expressive systems are always non-categorical (having unintended twins). Set theory is not fully categorical, but Peano Arithmetic is. Almog's main anti-essentialist argument.