Full Idea
Saying that the axiom of reducibility is logically necessary is what would be meant by saying that it is true in all possible worlds.
Gist of Idea
If something is true in all possible worlds then it is logically necessary
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XVII)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.193
A Reaction
This striking remark is a nice bridge between Leibniz (about whom Russell wrote a book) and Kripke.