Therefore, if some great idea takes hold of us from outside, we must understand that it takes hold of us only because something in us responds to it and goes out to meet it.  Richness of mind consists in mental receptivity, not in the accumulation of possessions.  What comes to us from outside, and, for that matter, everything that rises up from within, can only be made our own if we are capable of an inner amplitude equal to that of the incoming content. 

Possession can be formulated as identity of the ego-personality with a complex.

A common instance of this is identity with the persona, which is the individual’s system of adaptation to, or the manner he assumes in dealing with, the world.  Every calling or profession, for example, has its own characteristic persona.  It is easy to study these things nowadays, when the photographs of public personalities so frequently appear in the press.  A certain kind of behaviour is forced on them by the world, and professional people endeavour to come up to these expectations.  Only, the danger is that they become identical with their personas – the professor with his text-book, the tenor with his voice.  Then the damage is done; henceforth he lives exclusively against the background of his own biography. 

When consciousness is incapable either of grasping or knowingly integrating a spontaneously emerging impulse (the lance) or content (the enemy who throws it), the individual will instead be unconsciously possessed by it.  TGL 210