Full Idea
In the preceding chapter we agreed, though without being able to find demonstrative reasons, that it is rational to believe that our sense-data are signs of an independent reality.
Gist of Idea
It is rational to believe in reality, despite the lack of demonstrative reasons for it
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 3)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.13
A Reaction
I wonder if Russell was the first to grasp this essential distinction. I suspect that three hundred years (1600-1900) were wasted in philosophy because they thought that everything rational had to be demonstrable. E.g. Hume on induction.