In a treatise on elementary theosophy the solar system may be reckoned as our universe and we shall have no need of considering more than a small fragment of even that. It is septenary in constitution, as may be seen in its vibrations expressed in color and sound. Beyond the seven colors of the prism we have only tints and outside the seven notes we can get only overtones or undertones. There are likewise seven planes in the system but less than half of them require our attention, for the evolutionary field of the human soul is the three lower planes, known as the physical, astral and mental. When the human being has outgrown them in evolution he passes on to superhuman evolution.

The word "plane," so often encountered in theosophical literature, should perhaps have some definition. It has a wide application and is used as a synonym for region, place, sphere or world. In referring to the physical plane the term embraces all we know of earth and sky and life through the physical senses.

There are seven planes in our solar system because of the seven different combinations of its ultimate atoms. Each plane consists of a totally different grade of matter than the next plane, but all have for their base the ultimate atom of the solar system. When modern science discovered, to its astonishment, that the physical atom was a composite body it confirmed the theosophical teaching that the ultimate physical atom was not the final point of division. Theosophy teaches that when the ultimate physical atom is disintegrated its particles become the coarsest matter of the next plane or region above it—the astral plane. The process repeated with astral matter results in driving its ultimate atom from the highest level of the astral plane or world to the lowest of the mental plane. That scientist who said that the atom is the brick of the universe stated a great truth, for of its combinations all forms are built; and if the idea be applied to the ultimate atom of the solar system it will then be true that of such "bricks" all the planes are built.

The relationship of the planes to each other is that of interpenetrating spheres of matter. The physical plane, consisting of the earth and its atmosphere, is surrounded and interpenetrated by the astral plane, or world, which is an enormously larger globe of exceedingly tenuous matter. This vast sphere of invisible matter is within the earth as well as beyond it, interpenetrating every atom of physical matter to the earth's center. Its grossest grade of matter is so rare, and its vibrations so intense, that they cannot affect the physical senses and therefore we remain unconscious of it while that matter moves freely through all physical objects. We are unconscious of its life and activities for precisely the same reason that we know nothing of the messages of intelligence carried on the vibrations of the wireless telegraph, although they pass through the room where we sit. We have no sense organs with which it is possible to register such vibrations. Messages conveying intelligence of tremendous import, involving the movements of vast armies, the fall of empires and the destinies of great nations, flow through the very space we occupy but we are wholly unconscious of them. Even so we remain blind and deaf to the stupendous activities of life and consciousness in the astral world, notwithstanding the fact that it surrounds and permeates us while its forms, unseen and unfelt, move through the physical world as freely as water flows through a sieve.

The mental world constitutes a region of our earth still more vast than the astral portion of it. As the astral sphere encloses the physical globe, the mental encompasses both, enclosing them and also interpenetrating them to the earth's center. The term "mental world" may seem confusing to some because we are accustomed to think of the mental and the material as being opposites. The mental world, or sphere, or plane, of theosophy, is a world of matter, not merely thought. It is matter, however, of such remarkable tenuosity that it may properly be called mind-stuff, and in its rarest levels it is said to be "formless" so far as the existence of what the physical senses know as form is concerned.

All three of these worlds, or planes—the physical, astral and mental—are, then, worlds of matter, of form, of activity, of thought and of enterprise. They are concentric globes, the physical enclosed by the astral, and both physical and astral enclosed by the mental. Within and without all physical matter are both astral and mental matter. Every physical atom is surrounded and permeated by astral and mental matter. The relationship is precisely that which exists between the ether and the lower grades of physical matter.

If the relationship of the three worlds—physical, astral and mental—is fully understood later confusion of thought will be avoided. Physical language is not capable of fully expressing much with which students of the occult must deal. Because there is nothing better for the purpose, words must be used that express but a part of the truth and may sometimes prove misleading unless the constitution and relationship of the three spheres is kept in mind. Thus, it is necessary to speak of higher and lower worlds, or planes, inner or outer, and of the soul coming "down" into the material world when, as a matter of fact, no movement in space is under consideration. The astral is commonly spoken of as an inner plane and while it truly is so because it can be known only to astral senses by a withdrawal of the consciousness from its exterior, material body, it is also true that the astral world is outside the physical because it envelops it as the sea does a sponge. We usually speak of coming down from higher planes to lower and that may be true not only in the sense of changing the state of consciousness from higher vibrations to lower ones but it could mean a journey in space from a point in the astral plane above the physical globe to a point at its surface. "Up" and "down" are relative, not absolute. "Down" for us is toward the earth's center and "up" is the opposite direction. A spire in the Occident and a spire in the Orient are both said to be pointing upward but they are pointing in opposite directions. On most parts of the earth's surface we have four directions, while at the poles there is, of course, but one direction—south or north, as the case may be. East, west and north disappear at the north pole. Reflection upon such facts leads one to at least faintly comprehend the possibility of space itself disappearing from the inner planes—space as we know it.

The matter of each of the planes consists of seven classes. We are familiar with the solids, liquids and gases of the physical plane, and to them must be added four grades of the ether. The seven grades of matter of the astral and mental worlds constitute an important part of the mechanism for the soul's evolution, for they determine the state of consciousness in the life beyond the physical plane. But a study of those states of consciousness belongs to a later chapter.

A difficulty which the student of theosophy should make an early effort to eliminate, is the tendency to think of invisible realms as unreal. It should not be forgotten that it is only the limitation of the physical senses that gives rise to the feeling of unreality beyond the visible. We should keep in mind the fact that the invisible realms are composed of matter as certainly as the air is matter, or a stone is matter. The water in a pan may evaporate, but it does not cease to be matter because it has passed beyond the ken of the physical senses. It will some time condense once more and play its part as the liquid, water, or as the solid, ice. Only when matter is in certain forms can we know of its existence through the physical senses.

We frequently hear people who are students of the occult speak of a deceased person as having left the earth. But passing into the astral plane, or world, is not, of course, leaving the earth. Both the astral world and the mental world are divisions of the earth. As the atmosphere is invisible and yet is a part of the earth's physical matter, so the invisible astral and mental regions are other parts of the earth. They are properly called worlds because the activities in consciousness that make up existence there are as remote from ours as though they were upon another planet. We have erroneously supposed that with the physical senses we really see and know the earth, whereas we have known only that small fragment of the earth that consists of physical matter. Beyond the limitation of our poor senses stretch in unsuspected grandeur vaster regions of our earth, swept by the vibrations of an intenser life.