Nay, more than this. The theory of possession actually supplies us with a powerful method of co-ordinating and explaining many earlier groups of phenomena, if only we will consent to explain them in a way which at first sight seemed extreme in its assumptions--seemed unduly prodigal of the marvellous. Yet as to that difficulty we have learnt by this time that no explanation of psychical phenomena is really simple, and that our best clue is to get hold of some group which seems to admit of one interpretation only, and then to use that group as a _point de repère_ from which to attack more complex problems. Now I think that the Moses-Piper group of trance-phenomena cannot be intelligently explained on any theory except that of possession. And I therefore think it important to consider in what way earlier phenomena have led up to possession, and in what way the facts of possession, in their turn, affect our view of these earlier phenomena. If we analyse our observations of possession, we find two main factors--the central operation, which is the control by a spirit of the sensitive's organism; and the indispensable prerequisite, which is the partial and temporary desertion of that organism by the percipient's own spirit. Let us consider first how far this withdrawal of the living man's spirit from his organism has been rendered conceivable by evidence already obtained. First of all, the splits, and substitutions of phases of personality with which our second chapter made us familiar have great significance for _possession_ also. We have there seen some secondary personality, beginning with slight and isolated sensory and motor manifestations, yet going on gradually to complete predominance,--complete control of all supraliminal manifestation. The mere collection and description of such phenomena has up till now savoured of a certain boldness. The idea of tracing the possible mechanism involved in these transitions has scarcely arisen. Yet it is manifest that there must be a complex set of laws concerned with such alternating use of brain-centres;--developments, one may suppose, of those unknown physical laws underlying ordinary memory, of which no one has formed as yet even a first rough conception.