It has already been pointed out that Newton endeavoured to account for Gravitation by the pressure of the Aether. If, therefore, Gravitation be really due to this universal medium it becomes necessary to ask ourselves, What are the properties and characteristic qualities of this wonderful medium? What then is Aether, and what its properties? It has already been pointed out in Art. 29 that Aether is matter. Such an assumption is strictly in accordance with the Rules of Philosophy, quoted in Chap. I. Not only is this hypothesis a simple one, but it is also in accord with all our experience and observation. It is a simple supposition, because, unless Aether is assumed to be matter, then, instead of the universe being composed of two classes of things, matter and motion, we have to add a third class, which we call Aether. It can be readily seen, that by the introduction of a third class into the composition of the universe, such an addition, instead of simplifying the constitution of the universe, adds greater complexity to the same. By accepting the hypothesis that Aether is matter, we do away with the third class of essentials in the universe, and so reduce the number to two classes. If we could go one step further, and prove that instead of there being two classes of things in the universe, there was only one group, and show that all material things, and all phenomena could come under the head of either matter, or motion, then we should have reduced the universe to the simplest conception possible. As, however, it is not possible, at least in our present state of knowledge, for us to come to this fundamental and simple hypothesis of unity for the entire universe, we must accept the next simpler solution, and affirm that the universe is composed of two classes of things, viz. matter and motion, and this as I have already shown is a simpler classification than by putting Aether into a class by itself, and therefore is in accord with our first Rule of Philosophy.