In a "thought-transference " experiment of the normal type, the percipient's image or idea of a card or diagram is due (as we hold) to the fact that the agent has been directing his attention to that very image or idea. But in the case of these will-produced phantasms, the agent has not been picturing his own visible aspect. So far as he has been thinking of himself at all, it has been not of his aspect particularly, but of his personality, and of his personality in relation to the percipient. It is thus probable that the percipient's aspect has formed a larger part of the agent's whole idea than his own; yet it is his aspect, and nothing else, that is telepathically perceived. And a similar departure from the normal experimental type will meet us again in the large majority of the spontaneous telepathic cases. In some of these, the content of the agent's mind, at the time when the percipient received some sensory impression of him, has been a forcible idea of the percipient, and of himself in relation to the percipient; in others, we shall find that even this bond was lacking, and that the percipient's impression cannot be even loosely identified with any part of the conscious contents of the agent's mind.

These facts have, no doubt, a very real theoretic importance: they reveal a certain incompleteness in the transition which I have been endeavouring to make. As long as the impression in the percipient's mind is merely a reproduction of that in the agent's mind, it is possible to conceive some sort of physical basis for the fact of the transference. The familiar phenomena of the transmission and reception of vibratory energy are ready to hand as analogies - the effect, for instance, of a swinging pendulum on another of equal length attached to the same solid support; or of one tuning-fork or string on another of the same pitch; or of glowing particles of a gas on cool molecules of the same substance. Still more tempting are the analogies of magnetic and electrical induction. A permanent magnet brought into a room will throw any surrounding iron into a similar condition; an electric current in one coil of wire will induce a current in a neighbouring coil; though here even the medium of communication is unknown. So it is possible to conceive that vibration-waves, or nervous induction, are a means whereby activity in one brain may evoke a kindred activity in another - with, of course, a similar correspondence of psychical impressions.

Even here, perhaps, the conception should rather be regarded as a metaphor than an analogy. We have only to remember that the effect of all the known physical forces diminishes with distance - whereas we shall find reason to think that, under appropriate conditions, an idea may be telepathi-cally reproduced on the other side of the world as easily as on the other side of the room. The employment, therefore, of words like force, impulse, impact, in speaking of telepathic influences, must not be held to imply the faintest suspicion of what the force is, or any hypothesis whatever which would co-ordinate it with the recognised forces of the material world. Not only, as with other delicate phenomena of life and thought, is the subjective side of the problem the only one that we can yet attempt to analyse: we do not even know where to look for the objective side. If there really is a physical counterpart to the fact of transmission -over and above the movements in the two brains which are the termini of the transmission - that counterpart remains wholly unknown to us.

But a much more serious difficulty in the way of any physical conception of telepathy presents itself as soon as we pass to the cases where the image actually present in the agent's mind is no longer reproduced in the percipient's. A is dying at a distance; B sees his form. We may perhaps trace a relation between the processes in their two minds; but it certainly does not amount to anything like identity or distinct parallelism. That being so, there can be no such simple and immediate concordance as we have supposed between the nervous vibrations of their two brains; and that being so, there is no obvious means of translating into physical terms the causal connection between their experiences. This difficulty will take a somewhat different aspect when we come later to consider the part which the mind's unconscious operations may bear in telepathic phenomena. We may see grounds for thinking that a considerable community of experience (especially in emotional relations) between two persons may involve nervous records sufficiently similar to retain for one another some sort of revivable affinity, even when the experience has long lost its vividness for conscious memory.

Meanwhile it is best to admit the difficulty without reserve, and to state in the most explicit way that in the rapprochement between experimental thought-transference and spontaneous telepathic impressions we are confining ourselves to the psychical aspect; we connect the phenomena as being in all cases affections of one mind by another, occurring otherwise than through the recognised channels of sense. The objector may urge that if we have not, we ought to have, a physical theory which will embrace all the phenomena - that we ought not to talk about a rapport between A's mind and B's unless we can establish a bridge between their two brains. This seems rather to assume that the standing puzzle of the relation between cerebral and psychical events in the individual, B, can only be stated in one crude form - viz., that the former are prior and produce the latter; and though for ordinary purposes such an expression is convenient, the convenience has its dangers. Still, as the converse proposition - that the psychical events are the prior - would be equally dangerous, a crux remains which we cannot evade.

Since we cannot doubt that B's unwonted experience has its appropriate cerebral correlate, we have to admit that the energy of B's brain is directed in a way in which it would not be directed but for something that has happened to A. In this physical effect it is impossible to assume that an external physical antecedent is not involved; and the relation of the antecedent to the effect is, as I have pointed out, hard to conceive, when the neural tremors in A's brain are so unlike the neural tremors in B's brain as they must presumably be when A's mind is occupied with his immediate surroundings, or with the idea of death, and B's mind is occupied with a sudden and unaccountable impression or vision of A.

But however things may be on the physical plane, the facts recorded in this book are purely psychical facts; and on the psychical plane it is possible to give to a heterogeneous array of them a certain orderly coherence, and to present them as a graduated series of natural phenomena. Can it be asserted that this treatment is illegitimate unless a concurrent physical theory can also be put forward? It is surely allowable to do one thing at a time. There is an unsolved mystery in the background; that we grant and remember; but it need not perpetually oppress us. After all, is there not that standing mystery of the cerebral and mental correlation in the individual - a mystery equally unsolved and perhaps more definitely and radically insoluble - at the background of every fact and doctrine of the recognised psychology? The psychologists work on as if it did not exist, or rather as if it were the most natural and intelligible thing in the world, and no one complains of them. All that we claim is a similar freedom.