Full Idea
Upward Löwenheim-Skolem: if a set of first-order formulas is satisfied by a domain of at least the natural numbers, then it is satisfied by a model of at least some infinite cardinal.
Gist of Idea
Up Löwenheim-Skolem: if natural numbers satisfy wffs, then an infinite domain satisfies them
Source
Stewart Shapiro (Higher-Order Logic [2001], 2.1)
Book Reference
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.34