Full Idea
Where the idealist says that to be (i.e. to exist) is to be perceived, the phenomenalist says that to be is to be perceivable.
Gist of Idea
The phenomenalist says that to be is to be perceivable
Source
Cardinal/Hayward/Jones (Epistemology [2004], Ch.4)
Book Reference
Cardinal/Hayward/Jones: 'Epistemology: the theory of knowledge' [John Murray 2004], p.113
A Reaction
This is a nice phenomenalist slogan to add to Mill's well known one (Idea 3583). Expressed in this form, it looks false to me. What about neutrinoes? They weren't at all perceivable until recently. Maybe some physical stuff can never be perceived.
Related Idea
Idea 3583 External objects are permanent possibilities of sensation [Mill]