When uncertainty comes from the past: from the precautionary principle to the presumption principle. The sociology of collective risks has undergone an important renewal for the past fifteen years. Differing from approaches which considered the perception of risks, the most innovating works in this field have regarded collective risks according to a larger point of view: that of public problems whose emergence, framework and institutional treatment need to be questioned. These works notably attempted to analyze the changes induced by the acknowledgement of the scientific uncertainties regarding certain future threats. But uncertainty can come from the past too. It is the case when a latent period make a causal association between a risk exposure and a disease very difficult to establish. The example of French "atomic veterans", who raise today the question of health impact of nuclear testing, is taken to analyse the situations of uncertainty associated to the past. The paper shows that in this kind of situation, one of the main issue of the controversy lies on the application of a principle which looks like the precautionary principle but which is turned towards the past: the presumption principle.
