Using a couple of Paul Watzlawick's clinical cases as a starting point, the author shows how prescriptive behavioral strategies do not produce predictable effects: the theory of (nonlinear) complex systems prevents us from establishing a precise connection between a so-called psychotherapeutic act and what we consider therapeutic effects. It is precisely the consideration of the "Lorenz attractors" that thus brings us to reconsider the long psychoanalytic work as the condition for a general structural change of subjectivity: the result of this (...) work is a change of the neurotic "strange attractor.". (shrink)