Full Idea
The general proposition 'All products of two integers, which never have been and never will be thought of by any human being, are over 100' is undeniably true, and yet we can never give an instance of it; ..only a knowledge of the universals is required.
Clarification
'Integers' are whole numbers
Gist of Idea
We can know some general propositions by universals, when no instance can be given
Source
Bertrand Russell (Problems of Philosophy [1912], Ch.10)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.62
A Reaction
A nice example which it seems to be impossible to contradict. But maybe we can explain our knowledge of it in terms of rules, instead of mentioning universals. Can a rule be stated without recourse to universals? Sounds unlikely.