Full Idea
Sentences are types, not utterance tokens or mental/neural tokens, and hence sentences are abstract objects (like musical scores).
Gist of Idea
Sentences are abstract types (like musical scores), not individual tokens
Source
Jerrold J. Katz (Realistic Rationalism [2000], Int.xxvi)
Book Reference
Katz,Jerrold J.: 'Realistic Rationalism' [MIT 2000], p.-9
A Reaction
If sentences are abstract types, then two verbally indistinguishable sentences are the same sentence. But if I say 'I am happy', that isn't the same as you saying it.