### Abstract

We formalize a notion of logic whose terms and formulas are kept
abstract. In particular, logical connectives, substitution, free
variables, and provability are not defined, but characterized by their
general properties as locale assumptions. Based on this abstract
characterization, we develop further reusable reasoning
infrastructure. For example, we define parallel substitution (along
with proving its characterizing theorems) from single-point
substitution. Similarly, we develop a natural deduction style proof
system starting from the abstract Hilbert-style one. These one-time
efforts benefit different concrete logics satisfying our locales'
assumptions. We instantiate the syntax-independent logic
infrastructure to Robinson arithmetic (also known as Q) in the AFP
entry Robinson_Arithmetic
and to hereditarily finite set theory in the AFP entries Goedel_HFSet_Semantic
and Goedel_HFSet_Semanticless,
which are part of our formalization of Gödel's
Incompleteness Theorems described in our CADE-27 paper A
Formally Verified Abstract Account of Gödel's Incompleteness
Theorems.