Löwenheim–Skolem Theorem (Countable Model Form)
In a countable first-order language, a satisfiable theory has a model with at most countably many elements. Skolem's paradox distinguishes an external enumeration from a bijection available inside that model.
Before opening the references
first-order structure · Completeness of First-Order Logic · Countable Set
Where to read
Start here: Sets, Logic, Computation: An Open Introduction to Metalogic
Richard Zach; based on the Open Logic Text by the Open Logic Project · Open textbook · English · Free full text
§12.11 The Löwenheim–Skolem Theorem, Theorem 12.32 (finite or countably infinite model) and Example 12.34 (Skolem's Paradox)