To understand the methods by which a large class of physical phenomena are produced, it is necessary to have some comprehension of the various resources mentioned above, which a person functioning on the astral plane finds at his command; and this is a branch of the subject which it is by no means easy to make clear, especially as it is hedged about with certain obviously necessary restrictions. It may perhaps help us if we remember that the astral plane may be regarded as in many ways only an extension of the physical, and the idea that matter may assume the etheric state (in which, though intangible to us, it is yet purely physical) may serve to show us how the one melts into the other. In fact, in the Hindu conception of Jagrat, or "the waking state," the physical and astral planes are combined, its seven subdivisions corresponding to the four conditions of physical matter, and the three broad divisions of astral matter explained above. With this thought in our minds it is easy to move a step further, and grasp the idea that astral vision, or rather astral perception, may from one point of view be defined as the capability of receiving an enormously increased number of different sets of vibrations. In our physical bodies one small set of slow vibrations is perceptible to us as sound; another small set of much more rapid vibrations affects us as light; and again another set as electric action: but there are immense numbers of intermediate vibrations which produce no result which our physical senses can cognize at all. Now it will readily be seen that if all, or even some only, of these intermediates, with all the complications producible by differences of wave-length, are perceptible on the astral plane, our comprehension of nature might be very greatly increased on that level, and we might be able to acquire much information which is now hidden from us. [Sidenote: Clairvoyance.] [Sidenote: Prevision and Second-sight.]