Full Idea
The number of logical constants is not great: it appears, in fact, to be eight or nine.
Gist of Idea
There seem to be eight or nine logical constants
Source
Bertrand Russell (The Principles of Mathematics [1903], §012)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.11
A Reaction
There is, of course, lots of scope for interdefinability. No one is going to disagree greatly with his claim, so it is an interesting fact, which invites some sort of (non-platonic) explanation.