Full Idea
To grasp what it is to be red is to know the kind of sensory experience red things produce; ...but it is not true that to grasp what it is to be square one needs to know what kinds of sensory experience square things produce.
Gist of Idea
You understood being red if you know the experience involved; not so with thngs being square
Source
Colin McGinn (Subjective View: sec qualities and indexicals [1983], 8)
Book Reference
McGinn,Colin: 'The Subjective View' [OUP 1983], p.138
A Reaction
Are any experiences involved in the understanding of squareness? We don't know squareness by a priori intuition (do we?). To grasp squareness if may be necessary to have a variety of experiences of it. Or to grasp that it is primary.