Full Idea
If external objects be perceived immediately, we have the same reason to believe their existence as philosophers have to believe the existence of ideas.
Gist of Idea
The existence of ideas is no more obvious than the existence of external objects
Source
Thomas Reid (Essays on Intellectual Powers 6: Judgement [1785], 5)
Book Reference
Reid,Thomas: 'Inquiry and Essays', ed/tr. Beanblossom /K.Lehrer [Hackett 1983], p.273
A Reaction
He doesn't pay much attention to mirages and delusions, but in difficult conditions of perception we are confident of our experiences but doubtful about the objects they represent.