Instinctual drives, which are are conceptually central in psychoanalytic theory, underwent considerable elaboration and change over the long years of psychoanalysis. Freud's earliest theories on sexuality were designed to explain hysterical symptoms as an excitation in the nervous system. And this assumption expressed in terms of excitation remained important in his later theories of instinctual drives. In The Three Essays on the Theory of Sexuality, Freud established a set of hypotheses presenting human sexuality mainly as an expression of a sexual instinctual drive. Theoretical consideration of aggression appeared relatively late in Freud's work. And also, acceptance of the aggression as an instinctual drive parallel to libido always remains a problem in his studies. After Freud, Hartmann broadened the classical approach to the drive theory and suggested that role of agression as an independent source of energy has equal weight to the role of libido.