Proportion is the measure of relative quantity.

Since all quantity is divisible, it is evident that every distinct part into which any quantity is divided must bear some relation to the other parts, or to the whole.

These relations give an origin to the idea of proportion.
They are discovered by mensuration, and they are the objects of mathematical inquiry.

But whether any part of any determinate quantity be a fourth, or a fifth, or a sixth, or a moiety of the whole; or whether it be of equal length with any other part, or double its length, or but one half, is a matter merely indifferent to the mind; it stands neuter in the question: and it is from this absolute indifference and tranquillity of the mind, that mathematical speculations derive some of their most considerable advantages; because there is nothing to interest the imagination; because the judgment sits free and unbiassed to examine the point.

All proportions, every arrangement of quantity, is alike to the understanding, because the same truths result to it from all; from greater, from lesser, from equality and inequality.