47. In obsession the spirit acts exteriorly by the aid of its perispirit, which he mingles with that of the incarnates. The latter finds himself bound as if in a network, and constrained to act against his will.
In taking possession of a human organism, the free spirit substitutes itself, as it were, for that of the incarnated one, instead of acting exteriorly; he chooses a home in his body, although its owner does not leave it entirely, which can take place only with death. The possession is then only temporary and intermittent, for a discarnate has not the power to take exclusive possession of a human organism, only when the molecular union of the perispirit and body can be performed at the moment of conception (chap. XI, n° 18).
The spirit in momentary possession of the body uses it as his own. He speaks through its mouth, sees with its eyes, moves its arms as he had done in life. It is not as in mediumship when the incarnate speaks the thoughts of a discarnate, which are transmitted through him. It is the latter who speaks; and, if one has known him in life, one recognizes him by his language, voice, and gestures, even to the expression of his face.