The psychodynamic understanding of the personality and its disorders rests
heavily upon the study of the life cycle. The commonalities and similarities in
the course of all lives make possible the generalizations and abstractions
necessary for the scientific study of the personality. Although no two persons
are identical and no life stories are the same, the basic themes are limited and
it is the variations upon them that are infinite and inexhaustible. Behind their
individual uniqueness all persons are born with physical endowments that
are essentially alike and with similar biological needs that must be met.