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Full Idea
The notions of infinity and countability can be characterized by second-order sentences, though not by first-order sentences (as compactness and Skolem-Löwenheim theorems show), .. as well as well-ordering, progression, ancestral and identity.
Gist of Idea
Many concepts can only be expressed by second-order logic
Source
George Boolos (On Second-Order Logic [1975], p.48)
Book Ref
Boolos,George: 'Logic, Logic and Logic' [Harvard 1999], p.521
| 14249 | Boolos reinterprets second-order logic as plural logic [Boolos, by Oliver/Smiley] |
| 13841 | Why should compactness be definitive of logic? [Boolos, by Hacking] |
| 10829 | A sentence can't be a truth of logic if it asserts the existence of certain sets [Boolos] |
| 10830 | Second-order logic metatheory is set-theoretic, and second-order validity has set-theoretic problems [Boolos] |
| 10832 | '∀x x=x' only means 'everything is identical to itself' if the range of 'everything' is fixed [Boolos] |
| 10833 | Many concepts can only be expressed by second-order logic [Boolos] |
| 10834 | Weak completeness: if it is valid, it is provable. Strong: it is provable from a set of sentences [Boolos] |