If the Microcosm is the reproduction in ourselves of the same principles as exist in the Macrocosm or universe in which we have our being, then by investigating ourselves we shall learn the nature of the corresponding invisible principles in our environment.

Here, then, is the application of the dictum of the ancient philosophy, "Know Thyself." It means that the only place where we can study the principles of the invisible side of Nature is in ourselves; and when we know them there we can transfer them to the larger world around us.
In the concluding chapters of my "Edinburgh Lectures on Mental Science" I have outlined the way in which the soul or mind operates upon the physical instrument of its expression, and it resolves itself into this--that the mental action inaugurates a series of vibrations in the etheric body which, in their turn, induce corresponding grosser vibrations in the molecular substance until finally mechanical action is produced on the outside.

Now transferring this idea to Nature as a whole we shall see that if our mental action is to affect it in any way it can only be by the response of something at the back of material substance analogous to mind in ourselves; and that there is such a "something" interior to the merely material side of Nature is proved by what we may call the Law of Tendency, not only in animals and plants, but even in inorganic substances, as shown for instance in Professor Bose's work on the Response of Metals.

The universal presence of this Law of Tendency therefore indicates the working of some non-material and, so to say, semi-intelligent power in the material world, a power which works perfectly accurately on its own lines so far as it goes, that is to say in a generic manner, but which does not possess that Personal power of _individual selection_ which is necessary to bring out the infinite possibilities hidden in it.

This is what is meant by the Soul of Nature, and it is for this reason I employ that term instead of saying the material universe.