A contradiction runs through Freud's writings like a fault line. It arises form the fact that psychoanalysis presents itself as knowledge of two different kinds. On the one hand, psychoanalysis takes the form of an understanding of mind obtained through the disclosure of hidden meanings in dreams and neurotic symptoms. On the other, psychoanalysis takes the form of an explanation of mind secured in the elucidation of the mechanisms and systemic relations of a 'mental apparatus.'Freud, of course, straddled the line."
To bring these two forms of knowledge together within one science is like trying to square a circle. For they carry with them visions of mind that are fundamentally at odds. In seeking to understand mind through interpretation of meaning, Freud takes the mental as a property of a subject and his inner life. In seeking to explain mind as a mechanism, he places mental phenomena among the natural objects of the external world. Mind as meaning and mind as mechanism, however, lie on opposite sides of a great divide first enunciated by Descartes's famous dualism, in the distinction between res cogitans and res extensa, consciousness and matter, subject and object.
Stan Draenos Freud's Odyssey: Psychoanalysis and the End of Metaphysics (1982) p.7
quoted in Albert Tauber Freud - The Reluctant Philosopher (2010) p.67