Full Idea
Objects of sense are called 'real' when they have the kind of connection with other objects of sense which experience has led us to regard as normal; when they fail this, they are called 'illusions'.
Gist of Idea
Objects are treated as real when they connect with other experiences in a normal way
Source
Bertrand Russell (Our Knowledge of the External World [1914], 3)
Book Reference
Russell,Bertrand: 'Our Knowledge of the External World' [Routledge 1993], p.93
A Reaction
This rests rather too much on the concept of 'normal', but offers an attractive coherence account of perception. Direct perceptions are often invoked by anti-coherentists, but I think coherence is just as much needed in that realm.