There appears to be considerable misunderstanding even among Theosophical students upon this question of the reality of the various planes of the universe. This may perhaps be partly due to the fact that the word "plane" has occasionally been very loosely used in our literature--writers speaking vaguely of the mental plane, the moral plane, and so on; and this vagueness has led many people to suppose that the information on the subject which is to be found in Theosophical books is inexact and speculative--a mere hypothesis incapable of definite proof. No one can get a clear conception of the teachings of the Wisdom-Religion until he has at any rate an intellectual grasp of the fact that in our solar system there exist perfectly definite planes, each with its own matter of different degrees of density, and that some of these planes can be visited and observed by persons who have qualified themselves for the work, exactly as a foreign country might be visited and observed; and that, by comparison of the observations of those who are constantly working on these planes, evidence can be obtained of their existence and nature at least as satisfactory as that which most of us have for the existence of Greenland or Spitzbergen. The names usually given to these planes, taking them in order of materiality, rising from the denser to the finer, are the physical, the astral, the devachanic, the sushuptic, and the nirvanic. Higher than this last are two others, but they are so far above our present power of conception that for the moment they may be left out of consideration. Now it should be understood that the matter of each of these planes differs from that of the one below it in the same way as, though to a much greater degree than, vapour differs from solid matter; in fact, the states of matter which we call solid, liquid, and gaseous are merely the three lowest subdivisions of the matter belonging to this one physical plane.