Full Idea
Some say that logical constants are those expressions that are defined by their proof-theoretic roles, others that they are the expressions whose semantic values are permutation-invariant, and still others that they are the topic-neutral expressions.
Gist of Idea
Define logical constants by role in proofs, or as fixed in meaning, or as topic-neutral
Source
Theodore Sider (Writing the Book of the World [2011], 10.3)
Book Reference
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.223
A Reaction
[He cites MacFarlane 2005 as giving a survey of this]