Full Idea
For Dummett the semantics of the metalanguage is the external and objective source of the justification of deduction.
Gist of Idea
Deduction is justified by the semantics of its metalanguage
Source
report of Michael Dummett (The Justification of Deduction [1973]) by Robert Hanna - Rationality and Logic 3.4
Book Reference
Hanna,Robert: 'Rationality and Logic' [MIT 2006], p.67
A Reaction
This is offered as an answer to the Lewis Carroll problem that justifying deduction seems to need deduction, thus leading to a regress. [There is a reply to Dummett by Susan Haack]