Again, the theory has been advanced that all minds are in touch in a sort of subterranean way--through their subliminal regions--just as all spokes of a wheel ultimately reach the hub, though each spoke is distinctive. In this way we could imagine an inter-connection taking place, of which we are quite unaware, under certain favourable conditions. To use an analogy somewhere employed by Professor James, our conscious minds are like the leaves of the trees which whisper together, but the roots of the trees are all embedded in the same soil and are interlaced inextricably. So our minds, though they appear to be so separate and apart, may really be at basis fundamentally _one_. There must be, it is said, some common ground of interaction; possibly a sort of universal fluid, in which all minds are bathed, and by means of which interaction of thought is effected. This is somewhat akin to the theory first propounded by Mesmer, and which has been revived, in somewhat altered form, more than a hundred years later. Mesmer held that thought was communicated from brain to brain "by the vibrations of a subtle fluid with which the nerve substance is in continuity." Truly, if any sort of physical action is employed, this seems a significant enough remark. We know that two tuning forks will resound in unison, if one of them be struck. Put in motion a magnetized needle; at a certain distance and without contact another magnetized needle will oscillate synchronously with the first. Set in vibration a violin string, or the string of a piano; and at a certain distance the string of another piano or violin will vibrate in unison with it. Such analogies make us wonder whether or not communication of this kind might not exist, and, certainly, in order to make telepathy intelligible at all, we must suppose some such action taking place. We all have a tendency to think in physical symbols, owing to our materialistic training.