PROJECTION means the expulsion of a subjective content into an object; it is the opposite of introjection (q.v.). Accordingly it is a process of dissimilation (v. Assimilation), by which a subjective content becomes alienated from the subject and is, so to speak, embodied in the object. The subject gets rid of painful, incompatible contents by projecting them, as also of positive values which, for one reason or another-self-depreciation, for instance-are inaccessible to him. Projection results from the archaic identity (q.v.) of subject and object, but is properly so called only when the need to dissolve the identity with the object has already arisen. This need arises when the identity becomes a disturbing factor, i.e., when the absence of the projected content is a hindrance to adaptation and its withdrawal into the subject has become desirable. From this moment the 632 previous partial identity acquires the character of projection. The term projection therefore signifies a state of identity that has become noticeable, an object of criticism, whether it be the self-criticism of the subject or the objective criticism of another.
— Carl Gustav Jung
Projection is not, in Jung's account, a mistake that a sharper mind would avoid. It follows necessarily from archaic identity — the original condition in which subject and object have not yet separated enough to know themselves as distinct. The infant does not project onto the mother; the infant and the mother are not yet two things. What we call projection only becomes projection retroactively, when the merger has become a problem, when the shape of the object starts to distort under the weight of what we have placed there and adaptation begins to fail.
That retroactive quality deserves more attention than it usually gets. The withdrawal of a projection is not a correction of an error but a recognition of a boundary that was never firmly drawn in the first place. And notice what Jung says is projected: not only the painful and the incompatible, but also positive values — worth, beauty, power, the capacity for love — that have become inaccessible to the subject through self-depreciation. The object does not just receive what we cannot bear; it receives what we cannot yet claim. Projection is the soul's way of keeping something alive in the world when it cannot yet be housed inside. The question is not how to stop projecting but what the projection is holding that the subject has not yet been able to carry.
Carl Gustav Jung·Psychological Types·1921