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bryjnar comments on Godel's Completeness and Incompleteness Theorems - Less Wrong
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Nitpick, the Lowenheim-Skolem Theorems arre not quite that general. If we allow languages with uncountably many symbols and sets of uncountably many axioms then we can lower bound the cardinality (by bringing in uncountably many constants and for each pair adding the axiom that they are not equal). The technically correct claim would be that any set of axioms either have a finite upper bound on their models, or have models of every infinite cardinality at least as large as the alphabet in which they are expressed.
It is!? Does anyone know a proof of Compactness that doesn't use completeness as a lemma?
Yes. Or, at least, I did once! That's the way we proved it the logic course I did. The proof is a lot harder. But considering that the implication from Completeness is pretty trivial, that's not saying much.