I have suggested calling the total personality which, though present, cannot be fully known, the self. The ego is, by definition, subordinate to the self and is related to it like a part to the whole. Inside the field of consciousness it has, as we say, free will. By this I do not mean anything philosophical, only the well-known psychological fact of "free choice," or rather the subjective feeling of freedom. But, just as our free will clashes with necessity in the outside world, so also it finds its limits outside the field of consciousness in the subjective inner world, where it comes into conflict with the facts of the self. And just as circumstances or outside events "happen" to us and limit our freedom, so the self acts upon the ego like an objective occurrence which free will can do very little to alter. It is, indeed, well known that the ego not only can do nothing against the self, but is sometimes actually assimilated by unconscious components of the personality that are in the process of development and is greatly altered by them.
— Carl Gustav Jung
Jung is careful here to strip free will of its philosophical glamour and leave it as something more modest and more honest: the subjective feeling of freedom, operative within a bounded field. That boundary is the self, and the self does not wait for the ego's consent. It acts — Jung's word is telling — like an objective occurrence, something that happens to the ego the way weather happens, or loss, or the dream that arrives without invitation and refuses the interpretation you preferred.
This is where the passage earns its difficulty. The ego's ordinary project is management: to be the author of its own story, to choose, to grow in chosen directions, to develop deliberately. And Jung is saying that the self is largely indifferent to that project. More than indifferent — capable of assimilating the ego, of altering it from beneath, of enrolling it in a development the ego did not select and may not, for a long time, even recognize. The personality in process of development is not the personality improving on its own terms. It is the personality being changed by something it cannot see whole.
What that implies for any aspiration to "know yourself" and then act from that knowledge is not consoling. The knower is itself among the things being rearranged.
Carl Gustav Jung·Aion: Researches into the Phenomenology of the Self·1951