Winnicott never ceased to insist on the founding role of the actual mother in infant development. The assertion underpinned his theoretical and clinical work and formed the backbone of his lifelong disagreements with Melanie Klein. Against her insistence on the primacy of instinctual processes, he held out for the constituting role of the mother's adaptive behavior. He argued that the infant (the potential human person) was born into a medium of maternal responses (the environment mother) that held and contained him through the period of absolute dependence. The flexibility of this medium-mother—her capacity to be what the infant needed—was radically fateful for infant development: it was crucial for the early integration of the self and a major factor in setting up the infant