Built on careful observations of young children is Melanie Klein's object relations theory. The emphasis of Freud is on the first four to six years of life, contrasting the accentuation of Klein to the first four to six months of life. Klein insists the drives of infants such as hunger or sex are directed to an object such as a vagina, penis, a breast. The relation to the breast of the child according to Klein is fundamental, serving as a prototype to relations to whole objects later in life such as father and mother. The infantile tendency of relating to partial objects gives their experiences a fantasy-like or unrealistic nature affecting interpersonal relations later in life. Hence, the tendency of Klein's ideas shifts the focus of psychoanalytic theory to the role early fantasy plays in the formation of interpersonal relationships from organically based stages of development. Klein is not the only theorist to speculate the importance of the early relationship between child and mother. The belief of Margaret Mahler is for the sense of identity of children to be dependent on a three step relationship with their mother. The initial step is for mother to cater to the basic needs of infants and develop a symbiotic and safe relationship with the powerful mother. Finally, infants will establish their own identity and individuality upon emerging from the protective circle of their mother. The theory of Heinz Kohut is for the sense of self to develop during infancy when they are treated as a unique person by not only parents but others too. Investigating the attachment of infants to their mother is also John Bowlby who also investigates the negative affects separation from the mother can pose on an infant. Finally, Mary Ainsworth along with her colleagues develop a method of measuring the type of attachment style an infant displays toward their caregiver.