Full Idea
In order that we have available an explanation of the qualities of objects we need to be able to conceive primary qualities as consisting in something other than powers to produce experiences.
Gist of Idea
To explain object qualities, primary qualities must be more than mere sources of experience
Source
Colin McGinn (Subjective View: sec qualities and indexicals [1983], 6 n 52)
Book Reference
McGinn,Colin: 'The Subjective View' [OUP 1983], p.107
A Reaction
I suppose if the qualities are nothing more than the source of the experiences, that is Kant's noumenon. Nothing more could be said. The seems to be a requirement for tacit inference here. We infer the interior of the tomato.