Splitting is defined as an unconscious process by which positive and negative impulses and feelings that are too difficult to be held together spring apart and become projected onto different people. The clear split between good and bad relieves the anxiety of doubt in the toddler's world. "Daddy is bad; Mummy is nice."... "I love Mummy, I don't like Daddy".
Parents are the first targets for the projection of our polarized feelings and passions. Other relatives, playmates, pets, dolls, friends, neighbors, office colleagues, and so on follow. Their real selves can be obscured from and by the projector in different ways. The person might unload a revolting mess from the past onto the projection target. That sets the scene for destructive anger. Often, however, the luggage one cannot bear to owe and carry contains the best parts of oneself. Such denied aspects of ourselves become wonderful and precious ornaments, the beautiful clothes with which one dresses up others. Since it obscures the reality of the other, this idealizing projection leads to destruction too. The roots of both destructive anger and destructive idealization are in the mechanism of splitting.
The phraseology of Klein (1955, 143) comes useful here: at the initial, unintegrated stage of psychic development "splitting is at its height and persecutory anxiety predominates". Klein explains the splitting of the object into a "good" and a "bad" object as the earliest, most primitive kind of defense against anxiety, especially in the paranoid-schizoid position, in the first three or four months of life, when it affects the perception of part-objects.
See also DEFENCE MECHANISM, DEFENSIVE EXCLUSION, DESTRUCTIVE ANGER, DESTRUCTIVE IDEALIZATION, EARLY DAMAGE