Full Idea
Quantities such as numbers and dimensions, and also shapes (which are the limits of quantities) can be considered without their sensible qualities, which is for them to be abstracted from sensible matter.
Gist of Idea
Numbers and shapes are abstracted by ignoring their sensible qualities
Source
Thomas Aquinas (Summa Theologicae [1265], Q85 Ad2)
Book Reference
Aquinas,Thomas: 'Treatise on Human Nature (Summa 1a 75-89)', ed/tr. Pasnau,Robert [Hackett 2002], p.160
A Reaction
His account relies on underlying substance, which is where quantity is to be found (presumably because a substance is the epitome of a unit).