Full Idea
Dreams are more or less suggested by what we call waking life, and are capable of being more or less accounted for on scientific principles if we assume that there really is a physical world.
Gist of Idea
Dreams can be explained fairly scientifically if we assume a physical world
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 2)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.11
A Reaction
This sounds a bit circular, since scientific principles depend entirely on the assumption that there is a physical world. No doubt if we assume fairies, 'fairy lore' will explain everything. 'Explanation' is the basic concept here.