Full Idea
If the clauses of Tarski's definition of truth are turned into axioms (as Davidson proposed) then a primitive binary predicate symbol for satisfaction is needed, as Tarski defined truth in terms of satisfaction. Standard language has a unary predicate.
Gist of Idea
To axiomatise Tarski's truth definition, we need a binary predicate for his 'satisfaction'
Source
Volker Halbach (Axiomatic Theories of Truth [2011], 5.2)
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.35