Full Idea
No proposition of logic can assert 'existence' except under a hypothesis.
Gist of Idea
Logic can only assert hypothetical existence
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XVIII)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.204
A Reaction
I am prepared to accept this view fairly dogmatically, though Musgrave shows some of the difficulties of the if-thenist view (depending on which 'order' of logic is being used).