This is equally true with respect to the astral body of man, which for our purpose at the moment we may regard as consisting of two parts—the denser aggregation which occupies the exact position of the physical body, and the cloud of rarer astral matter which surrounds that aggregation. In both these parts, and between them both, there is going on at every moment of time the rapid inter-circulation of the particles which has been described, so that as one watches the movement of the molecules in the astral body one is reminded of the appearance of those in fiercely boiling water.

This being so, it will be readily understood that though any given organ of the physical body must always have as its counterpart a certain amount of astral matter, it does not retain the same particles for more than a few seconds at a time, and consequently there is nothing corresponding to the specialization of physical nerve-matter into optic or auditory nerves, and so on. So that though the physical eye or ear has undoubtedly always its counterpart of astral matter, that particular fragment of astral matter is no more (and no less) capable of responding to the vibrations which produce astral sight or astral hearing than any other part of the vehicle.

It must never be forgotten that though we constantly have to speak of "astral sight" or "astral hearing" in order to make ourselves intelligible, all that we mean by those expressions is the faculty of responding to such vibrations as convey to the man's consciousness, when he is functioning in his astral body, information of the same character as that conveyed to him by his eyes and ears while he is in the physical body. But in the entirely different astral conditions, specialized organs are not necessary for the attainment of this result; there is matter in every part of the astral body which is capable of such response, and consequently the man functioning in that vehicle sees equally well objects behind him, beneath him, above him, without needing to turn his head.

There is, however, another point which it would hardly be fair to leave entirely out of account, and that is the question of the chakrams referred to above. Theosophical students are familiar with the idea of the existence in both the astral and the etheric bodies of man of certain centres of force which have to be vivified in turn by the sacred serpent-fire as the man advances in evolution. Though these cannot be described as organs in the ordinary sense of the word, since it is not through them that the man sees or hears, as he does in physical life through eyes and ears, yet it is apparently very largely upon their vivification that the power of exercising these astral senses depends, each of them as it is developed giving to the whole astral body the power of response to a new set of vibrations.

Neither have these centres, however, any permanent collection of astral matter connected with them. They are simply vortices in the matter of the body—vortices through which all the particles pass in turn—points, perhaps, at which the higher force from planes above impinges upon the astral body. Even this description gives but a very partial idea of their appearance, for they are in reality four-dimensional vortices, so that the force which comes through them and is the cause of their existence seems to well up from nowhere. But at any rate, since all particles in turn pass through each of them, it will be clear that it is thus possible for each in turn to evoke in all the particles of the body the power of receptivity to a certain set of vibrations, so that all the astral senses are equally active in all parts of the body.

The vision of the mental plane is again totally different, for in this case we can no longer speak of separate senses such as sight and hearing, but rather have to postulate one general sense which responds so fully to the vibrations reaching it that when any object comes within its cognition it at once comprehends it fully, and as it were sees it, hears it, feels it, and knows all there is to know about it by the one instantaneous operation. Yet even this wonderful faculty differs in degree only and not in kind from those which are at our command at the present time; on the mental plane, just as on the physical, impressions are still conveyed by means of vibrations travelling from the object seen to the seer.

On the buddhic plane we meet for the first time with a quite new faculty having nothing in common with those of which we have spoken, for there a man cognizes any object by an entirely different method, in which external vibrations play no part. The object becomes part of himself, and he studies it from the inside instead of from the outside. But with this power ordinary clairvoyance has nothing to do.

The development, either entire or partial, of any one of these faculties would come under our definition of clairvoyance—the power to see what is hidden from ordinary physical sight. But these faculties may be developed in various ways, and it will be well to say a few words as to these different lines.

We may presume that if it were possible for a man to be isolated during his evolution from all but the gentlest outside influences, and to unfold from the beginning in perfectly regular and normal fashion, he would probably develop his senses in regular order also. He would find his physical senses gradually extending their scope until they responded to all the physical vibrations, of etheric as well as of denser matter; then in orderly sequence would come sensibility to the coarser part of the astral plane, and presently the finer part also would be included, until in due course the faculty of the mental plane dawned in its turn.

In real life, however, development so regular as this is hardly ever known, and many a man has occasional flashes of astral consciousness without any awakening of etheric vision at all. And this irregularity of development is one of the principal causes of man's extraordinary liability to error in matters of clairvoyance—a liability from which there is no escape except by a long course of careful training under a qualified teacher.