Full Idea
To grasp what it is for something to be square it is not constitutively necessary to know how square things look or feel, since what it is to be square does not involve any such relation to experience.
Gist of Idea
You don't need to know how a square thing looks or feels to understand squareness
Source
Colin McGinn (Subjective View: sec qualities and indexicals [1983], 2)
Book Reference
McGinn,Colin: 'The Subjective View' [OUP 1983], p.8
A Reaction
You could even describe squareness verbally, unlike redness. It seems crucial that almost any sense (such as bat echoes) can communicate primary qualities, but secondary qualities are tied to a sense, and wouldn't exist without it.
Related Idea
Idea 22413 Being red simply consists in looking red [McGinn]