In the first place, there seems to be no definite or prescribed area in the brain adapted for such a purpose; no cell or centre has as yet been discovered which appears destined to send out waves of this character. Still, perhaps it will be some day, for the functions of certain portions of the brain--particularly the frontal lobes--are as yet very little understood. But there is the argument that, if such waves exist, they must be detected by means of our scientific instruments--instruments so delicate and subtle that they are able to measure the difference of the pull of gravity of an article when placed on the table or on the floor, or can register the heat of a candle at a distance of more than a mile (Langley's bolometer). Compared with such delicate instruments, our five senses are coarse indeed, and any vibrations which can affect these same senses must surely affect the more delicate and sensitive instruments just mentioned. Yet none of them have as yet been able to indicate the existence of any such vibrations, and this would seem to show that they cannot exist.[42] But there is a reply to this argument. It may be said that, although the _senses_ do not register any such vibrations, the _brain_ might do so, in some direct manner; and the brain might be far more sensitive than any instrument so far devised. Indeed the definition of telepathy, "the ability of one mind to influence or be influenced by another mind otherwise than through the recognized channels of sense," would seem to indicate that in this process only the brain is involved, and not necessarily the physical senses at all. So far, then, so good; telepathy might still be vibratory in character. But if so, how could such waves get through the skull to act upon the brain direct? This is a staggering thought to the ordinary materialist, and at first sight renders such an action unintelligible and hence "impossible"! But to reason thus would be very superficial. For we know that certain physical energies pass through solid substances--substances impervious to other physical energies. Thus we know that glass permits light to pass through it, but is a non-conductor of electricity; while steel is impervious to light, yet electricity can traverse miles of steel in the fraction of a second. "Gravity" seems the only energy which cannot be isolated by some means or other. No substance is opaque to gravity. It acts through all substances, at all times, continuously. In this respect telepathy may resemble gravitation.[43] If this were true, or anything like it were true, we could easily see why a solid substance, such as the human skull, might offer no appreciable resistance to the passage through it of undulations of a certain velocity--of a speed so great, perhaps, that they could not be detected by any of the instruments at the command of the physicist today.