Chiesa, L. (2007). Subjectivity and Otherness: A Philosophical Reading of Lacan. Massachusetts: MIT Press.
The discovery of a structural split in/of the subject subverts the Cartesian cogito while, at the same time, revealing its intimate relation to psychoanalysis. As Lacan repeatedly states, “I think where I am not, therefore I am where I do not think.” (39)
This is to say that the unconscious I, the subject of the enunciation, really thinks at the unconscious level: it (ça) thinks where the I qua ego and qua subject of the statement is not (conscious).
Conversely, the I qua linguistic ego is (conscious) where it, the unconscious subject, does not think.
Most importantly, the subversion of the Cartesian ego shows that its illusion of unity is possible only because of its strict interdependence with the Spaltung. There is no self consciousness without the unconscious, and vice versa.
Descartes’s formulation of a fundamental principle of self-consciousness could be said contemporaneously to decree, at the level of the history of thought, the formal birth of the unconscious (although this birth will remain implicit until Freud). Consequently, it is strictly speaking senseless to speak of a pre-Cartesian notion of the unconscious.