This article examines the ethics of ambiguity formulated by existentialist authors in the 1940s, linking it to turn-of-the-century debates on ethics between philosophy and the social sciences. The underlying thesis is that, rather than representing a radical conceptual novelty, the ethical thought of Jean-Paul Sartre and Simone de Beauvoir, strained between freedom and situation, constitutes a revisiting and updating of philosophical positions from the landscape of the Third Republic. To demonstrate this, the thought of Frédéric Rauh is examined. Indeed, anticipating (...) what de Beauvoir would later call “ambiguity”, Rauh sought to develop an ethics of action that would constitute a third way between sociological objectivism and the empty abstractions of idealism and rationalism. By reconstructing the transitions from one generation and historical context to the next, the article shows how, and through which mediators, the existentialists were well aware of this debate and, in their own way, developed a philosophy of action, of ethical experience, similar to Rauh’s. (shrink)