The paradox of self-realization versus social definition is an important consideration in theories of human identity. Freud linked self-realization to the drive of the instincts to find satisfaction against the opposition of the environment, which then exerted still further pressure against the instincts through the ego and superego. Hartmann and other ego psychologists indicated that identity formation need not be thought of as conflict between self-realization and social definition but stressed the synthesizing capacity of the ego to 'put together' biological and social pressures from the viewpoint of the self. Erikson postulated the concept of 'mutuality' between biological needs of the individual and the need of society but also indicated identity can exist apart from the principle of mutuality.