Full Idea
I believe that common sense is right in regarding what we see as physical and (in one of several possible senses) outside the mind, but is probably wrong in supposing that it continues to exist when we are no longer looking at it.
Gist of Idea
Visible things are physical and external, but only exist when viewed
Source
Bertrand Russell (The Ultimate Constituents of Matter [1915], p.123)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.123
A Reaction
This remark (in 1915) is a bit startling from a philosopher well known for his robustly realist stance. Just one of his phases! It seems very counterintuitive - that objects really exist externally, but only when viewed. Schrödinger's Cat?