[The Wikipedia page for Margaret Mahler barely hints at her contributions; the Margaret Mahler Foundation offers additional resources; The Psychological Birth of the Human Infant: Symbiosis and Individuation, written with Fred Pine and Anni Bergman, is an accessible introduction to her work.]
Once the infant begins to crawl (the Practicing phase) its nascent Individuation-Separation phase escalates. During this time, the infant is slowly recognizing that it is separate from its mother and beginning to develop its early primitive Theory of Mind. Parents appreciate this development, whose hallmark is the child’s adamant “No!” A child who can say “No!” is a child who has successfully traversed a significant developmental milestone and is ready to enter the next phases of development.
It is during this time that the child is developing Object Constancy. Object Constancy and its inverse, Splitting, are the key concepts to understanding BPD: