The case becomes still more striking when we have to deal with conceptions of the universe, of cosmic forces such as light and heat, or of the stupendous secular changes which modern science calls us to contemplate. Here our conceptions cannot even pretend to represent the objects; they are as purely symbolic as the algebraic equations whereby the geometer expresses the shapes of curves. Yet so long as there are means of verification at our command, we can reason as safely with these symbolic conceptions as if they were truly representative. The geometer can at any moment translate his equation into an actual curve, and thereby test the results of his reasoning; and the case is similar with the undulatory theory of light, the chemist's conception of atomicity, and other vast stretches of thought which in recent times have revolutionized our knowledge of Nature. The danger in the use of symbolic conceptions is the danger of framing illegitimate symbols that answer to nothing in heaven or earth, as has happened first and last with so many short-lived theories in science and in metaphysics. Forewarned of this danger, and therefore--I hope--forearmed against it, let us see what a scientific philosophy has to say about the Power that is manifested in and through the universe. XIII. _The Eternal Source of Phenomena._ We have seen that before men could arrive at the idea of God, before out of the old crude and fragmentary polytheisms there could be developed a pure and coherent theism, it was necessary that physical generalization should have advanced far enough to enable them, however imperfectly, to reason about the universe as a whole. It was a faint glimpse of the unity of Nature that first led men to the conception of the unity of God, and as their knowledge of the phenomenal fact becomes clearer, so must their grasp upon the noumenal truth behind it become firmer. Now the whole tendency of modern science is to impress upon us ever more forcibly the truth that the entire knowable universe is an immense unit, animated throughout all its parts by a single principle of life. This conclusion, which was long ago borne in upon the minds of prophetic thinkers, like Spinoza and Goethe, through their keen appreciation of the significance of the physical harmonies known to them, has during the last fifty years received something like a demonstration in detail. It is since Goethe's death, for example, that it has been proved that the Newtonian law of gravitation extends to the bodies which used to be called fixed stars. That such was the case was already much more than probable, but so lately as 1835 there were to be found writers on science, such as Comte, who denied that it could ever be proved. But a still more impressive illustration of the unity of Nature is furnished by the luminiferous ether, when considered in connection with the discovery of the correlation of forces. The fathomless abysses of space can no longer be talked of as