Full Idea
Using the definition of truth we are in a position to carry out the proof of consistency for deductive theories in which only (materially) true sentences are (formally) provable.
Gist of Idea
Using the definition of truth, we can prove theories consistent within sound logics
Source
Alfred Tarski (The Establishment of Scientific Semantics [1936], p.407)
Book Reference
Tarski,Alfred: 'Logic, Semantics, Meta-mathematics' [Hackett 1956], p.407
A Reaction
This is evidently what Tarski saw as the most important first fruit of his new semantic theory of truth.