Freud has often been accused of a crude determinism, since no slip or blunder, no matter how apparently insignificant, is ever ascribed to chance.
Indeed, Freud wrote, "I believe in external (real) chance, it is true, but not in internal (psychical) accidental events."
Lacan expresses the same belief in his own terms: chance, in the sense of pure contingency, only exists in the real.
In the symbolic order, there is no such thing as pure chance.
Automaton and Tyche
In the seminar of 1964, Lacan uses Aristotle's distinction between two kinds of chance to illustrate this distinction between the real and the symbolic.
In the second book of the Physics, where the concept of causality is discussed, Aristotle explores the role of chance and fortune in causality.
He distinguishes between two types of chance:
Lacan redefines automaton as "the network of signifiers", thus locating it in the symbolic order.
The term thus comes to designate those phenomena which seem to be chance but which are in truth the insistence of the signifier in determining the subject.
Automaton is not truly arbitrary: only the real is truly arbitrary, since "the real is beyond the automaton."
The real is aligned with tyche, which Lacan redefines as "the encounter with the real".
Tyche thus refers to the incursion of the real into the symbolic order: unlike the automaton, which is the structure of the symbolic order which determines the subject, tyche is purely arbitrary, beyond the determinations of the symbolic order.
It is a knock on the door that interrupts a dream, and on a more painful level it is trauma.
The traumatic event is the encounter with the real, extrinsic to signification.