[Henri Atlan's levels of ethics and the challenge of the "fourth level"]

This article presents the ideas on ethics by the contemporary French biophysicist and Spinozist philosopher Henri Atlan, based on his singular epistemological position, in constant transit between naturalistic philosophy (in alliance with cognitive sciences) and his refusal to a natural foundations of ethics. It underlines Atlan's defense of casuistry towards bioethics dilemmas and associates it to his proposal of different levels of ethics. The text introduces a reflection concerning Atlan's ideas about the possible passage between the third and the fourth levels of ethics, stressing its positive impact in individuals and social groups' life quality.