Theosophy is an Objective Idealism. It is an Idealism, but not a pure or complete idealism like Bishop Berkeley's philosophy or Christian Science or some of the Oriental philosophies which deny the existence of matter altogether and say, All is mind, there is no matter. Or, All is God, or Spirit, there is no matter, no objective world. Theosophy is more realistic than that. Theosophy says that the objective world is a Maya, that is it is not as real as it seems, though it is real enough to those beings who are enmeshed in it. During the period of manifestation or Manvantara the objective world is real to all the beings who are a part of the manifestation but as the manifestation proceeds on the Upward Arc of Evolution it is indrawn into the Divine Spirit from which it first emanated.
All religions and many of the philosophies (the spiritual ones) admit, or rather, affirm, that the fundamental principle of the universe is a spiritual principle, call it by whatever name you like, God, or Brahman, or Tao, Adi-Buddha, or a hundred other names. In any case it is as real to the spiritual consciousness as air and sunshine are real to our objective consciousness. Let us quit disputing over names and admit that a fundamental Truth, a fundamental Reality, underlies all religions and that the forms which the various religions and philosophies take are more or less perfect, more or less imperfect, efforts to explain the Truth to the people or the age to which it was given. Theosophy is the latest such effort. Theosophy will explain how the physical world is only a temporary, transitory thing, therefore called by philosophy unreal, a Maya, a deceptive thing, not just what it seems. Modern science should greatly help us to understand this point. In the laboratory a qualified scientist, with proper equipment, can demonstrate to us quite conclusively that this solid floor on which we stand is composed of atoms which can be shown to be only points of electrical energy, electrons and protons, with as much space between them as there is between the sun and the planets. This floor, this physical world, is real to us because we are composed of the same electrons and protons as it is. We are tuned in to it, we are in the same general or cosmic rate of vibration as it; therefore it seems real, it is tangible to us. Like the empty, soundless air, when we tune our radio into it we hear. If, like the clairvoyant, we could tune ourselves into a finer state of matter, the Astral Light, then the Astral Light would be the real to us. We should see right through physical walls. "Matter" would seem to us as a mere shadow. The astral would be the real world.
Now, suppose we could tune in to the next higher plane, the Manasic, the world of "mind stuff," Akasa. From there the astral world would seem to us as the great illusion it really is. Suppose again, that we tune in to the world of pure spirit, Buddhi, Christos. Then that, indeed, would be the real. And so on forever. But we have no words and no higher faculties for the divine infinities which may be beyond the spirit. We can say, if we wish, super-cosmic consciousness — Nirvana — the One Reality to the Nirvani, but an utter abstraction to us.
The point to remember is that whatever we are tuned in to, that, to us, for the time, is the real. Each time we wake up to something higher, that, to us, becomes the real. What is below us is left behind, an unreality. This does not mean that there is no reality. On the contrary, the only permanent, the only eternal part of us is the ever present, ever accessible, Divinity within.
So Theosophy is an Objective Idealism because it postulates a fundamental divine Reality and admits a seeming or temporary or transitory reality to all manifested planes as long as the manifestation (Manvantara) lasts.