Full Idea
We know that certain scientific propositions - often expressed in mathematical symbols - are more or less true of the world, but we are very much at sea as to the interpretation to be put upon the terms which occur in these propositions.
Gist of Idea
Mathematically expressed propositions are true of the world, but how to interpret them?
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], VI)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.55
A Reaction
Enter essentialism, say I! Russell's remark is pretty understandable in 1919, but I don't think the situation has changed much. The problem of interpretation may be of more interest to philosophers than to physicists.