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10833 | Many concepts can only be expressed by second-order logic [Boolos] |
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. | |
From: George Boolos (On Second-Order Logic [1975], p.48) |