The original concept of transitional phenomena was used by Donald Winnicott to describe the intermediate area of human experience between inner reality and the outside world. The prototypical example is that of the transitional object, the first not-me possession of the baby. Thus a real, usually soft object is found by the baby and used as a defense against anxiety.
The transitional object represents a real paradox in that it is not an internal object; it is a possession yet it is not an external object either. It grows out of the baby's relationship with the breast (which represents the whole technique of mothering) and his own body. For example, while sucking on his thumb he weaves not-me objects into his own autoerotic experience and personal pattern.