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.