If we did not know that what we call _quantity of substance_ is measured by the property named _mass_, we might very well accept the proposition that the entrance of phlogiston into a substance decreases the quantity, hence the mass, and, therefore, the weight, of the substance.
Although Stahl and his followers were emerging from the trammels of alchemy, they were still bound by many of the conceptions of that scheme of nature.

We have learned, in previous chapters, that the central idea of alchemy was expressed in the saying: "Matter must be deprived of its properties in order to draw out its soul." The properties of substances are everything to the modern chemist--indeed, such words as iron, copper, water, and gold are to him merely convenient expressions for certain definable groups of properties--but the phlogisteans regarded the properties of things, including mass, as of secondary importance; they were still trying to get beneath the properties of a thing, to its hypothetical essence, or substance.
Looking back, we cannot think of phlogiston as a substance, or as a thing, in the modern meanings of these terms as they are used in natural science.

Nowadays we think, we are obliged to think, of the sum of the quantities of all the things in the universe as unchanging, and unchangeable by any agency whereof we have definite knowledge.

The meaning we give to the word _thing_ rests upon the acceptance of this hypothesis.

But the terms _substance_, _thing_, _properties_ were used very vaguely a couple of centuries ago; and it would be truly absurd to carry back to that time the meanings which we give to these terms to-day, and then to brand as ridiculous the attempts of the men who studied, then, the same problems which we study now, to express the results of their study in generalisations which employed the terms in question, in what seems to us a loose, vague, and inexact manner.
By asserting, and to some extent experimentally proving, the existence of one principle in many apparently very different substances (or, as would be said to-day, one property common to many substances), the phlogistic theory acted as a very useful means for collecting, and placing in a favourable position for closer inspection, many substances which would probably have remained scattered and detached from one another had this theory not been constructed.