Full Idea
Psychologists point out that illusions almost always occur in unnatural environments in which subjects are prevented from exploiting the natural interplay between perception and action.
Gist of Idea
Psychologists say illusions only occur in unnatural and passive situations
Source
E.J. Lowe (Introduction to the Philosophy of Mind [2000], Ch. 6)
Book Reference
Lowe,E.J.: 'Introduction to the Philosophy of Mind' [CUP 2000], p.153
A Reaction
It has always struck me that philosophers make a great deal out of illusions, but I don't think I have ever had one. I don't know anyone who has seen a non-existent dagger.