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During its theoretical development, Parsons showed a persistent interest in symbolism. An important statement is Parsons' "The Theory of Symbolism in Relation to Action". The article was stimulated by a series of informal discussion group meetings, which Parsons and several other colleagues in the spring of 1951 had conducted with philosopher and semiotician Charles W. Morris. His interest in symbolism went hand in hand with his interest in Freud's theory and "The Superego and the Theory of Social Systems", written in May 1951 for a meeting of the American Psychiatric Association. The paper can be regarded as the main statement of his own interpretation of Freud, but also as a statement of how Parsons tried to use Freud's pattern of symbolization to structure the theory of social system and eventually to codify the cybernetic hierarchy of the AGIL system within the parameter of a system of symbolic differentiation. His discussion of Freud also contains several layers of criticism that reveal that Parsons' use of Freud was selective rather than orthodox. In particular, he claimed that Freud had "introduced an unreal separation between the superego and the ego".