Let c={c1,c2…} be a countable set.
Add the propositional function p(x)=(x=c1)∨(x=c2)∨…(infinitely) to the usual language.
The closed logical formulas are still countable. You can use Gödel's theorem.
Let X be an axiom system by the language.
Let d be a free individual symbol. The mathematical axioms in X don't include d.
Put X" =X ∪{∀xp(x)}.
Theorem 1. X" isn't consistent.
Proof. If X" is consistent, X"∪{d≠c1}∪…∪{d≠cm} has a model Mm for which
Mm |= d=cm+1 and is consistent for ∀m∈N.
So X"∪{d≠c1}∪{d≠c2}∪…(infinitely) is consistent and has a mdel M by Göel's theorem.
M |= d≠c1, M |= d≠c2,…(infinitely)
The object domain of M is larger than c. But M |= ∀xp(x). This is contradiction.♦
Add the propositional function p(x)=(x=c1)∨(x=c2)∨…(infinitely) to the usual language.
The closed logical formulas are still countable. You can use Gödel's theorem.
Let X be an axiom system by the language.
Let d be a free individual symbol. The mathematical axioms in X don't include d.
Put X" =X ∪{∀xp(x)}.
Theorem 1. X" isn't consistent.
Proof. If X" is consistent, X"∪{d≠c1}∪…∪{d≠cm} has a model Mm for which
Mm |= d=cm+1 and is consistent for ∀m∈N.
So X"∪{d≠c1}∪{d≠c2}∪…(infinitely) is consistent and has a mdel M by Göel's theorem.
M |= d≠c1, M |= d≠c2,…(infinitely)
The object domain of M is larger than c. But M |= ∀xp(x). This is contradiction.♦