Full Idea
For sentential or first-order logic, the logical truths are represented by valid formulas; in higher-order logics, by sentences formulated in purely logical terms.
Gist of Idea
In classical logic, logical truths are valid formulas; in higher-order logics they are purely logical
Source
Michael Dummett (Frege philosophy of mathematics [1991], Ch. 3)
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.26