Full Idea
Whoever denies the existence of truth grants that truth does not exist: and if truth does not exist, then the proposition 'Truth does not exist' is true: and if there is anything true, there must be truth.
Gist of Idea
If the existence of truth is denied, the 'Truth does not exist' must be true!
Source
Thomas Aquinas (Summa Theologicae [1265], Art 1, Obj 3)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.31
A Reaction
A classic example of turning the tables, also applicable to anyone who firmly denies knowledge, or that words are meaningful, or says that meaning needs verification. However, one measily truth is not much consolation.