more from George Boolos

Single Idea 10834

[catalogued under 5. Theory of Logic / K. Features of Logics / 4. Completeness]

Full Idea

A weak completeness theorem shows that a sentence is provable whenever it is valid; a strong theorem, that a sentence is provable from a set of sentences whenever it is a logical consequence of the set.

Gist of Idea

Weak completeness: if it is valid, it is provable. Strong: it is provable from a set of sentences

Source

George Boolos (On Second-Order Logic [1975], p.52)

Book Reference

Boolos,George: 'Logic, Logic and Logic' [Harvard 1999], p.525


A Reaction

So the weak version says |- φ → |= φ, and the strong versions says Γ |- φ → Γ |= φ. Presumably it is stronger if it can specify the source of the inference.