第 4 条 · 数是量对单位的比例

So that the determination, or the measure of magnitude of all kinds, is reduced to this: fix at pleasure upon any one known magnitude of the same species with that which is to be determined, and consider it as the measure or unit; then, determine the proportion of the proposed magnitude to this known measure.

所以，一切量的确定或度量都归结为这一点：任取一个与待定量同类的已知量，把它看作尺度，也就是单位；然后确定待定量同这个已知尺度之间的比例。

This proportion is always expressed by numbers; so that a number is nothing but the proportion of one magnitude to another arbitrarily assumed as the unit.

这种比例总是用数来表示；因此，所谓数，不过是一个量同另一个任意选作单位的量之间的比例。