Which knowledge can we trust in the field of psy- disciplines? Which one is so "strong" to be the starting point of reliable tools? And the results we get, are they talking about the same thing we wanted to know? Through the help of contributions coming from different disciplines, this article offers an overview on the evolution of contemporary epistemological structures underlying psychological investigation. What follows is a reflection on the "complex" relationship between scientific research, theory, diagnosis and treatment. Possible analog connections are then opened between different scientific fields where the concept of "chaotic attractor" stands as a possible liaison between the supporters of the standard cognitivism and radical constructivist positions.
Keywords: Diagnosis, complexity, epistemology, evidence-based, constructivism, RCT.