Uncertainty in law as in politics is increasingly mobilized in order to implement policy more quickly and more efficiently. As a consequence, social actors rely less and less on proven facts, reliable reasoning or indisputable arguments. It is the unknown or the uncertain that becomes the basis for political mobilization. The French case supports this observation, particularly with regard to recent policies in the fight against terrorism. In the formulation and legitimation of anti-terrorism policy in the wake of the attacks on the Promenade des Anglais in Nice on July 14, 2016, one can observe a significant change in the discourse of justice and home affairs from uncertainty as a unfortunate contingency to be dealt with or neutralised by certain means, to uncertainty as the very legitimating rationale of political action. Uncertainty justifies, inspires and mobilizes. Building upon documentation and analysis of recent evolution in French anti-terror legislation in this area, this paper argues that the opposition between "certainty” and “uncertainty", like that between "security" and "insecurity", is not symmetrical, but rather that the potential force of uncertainty varies as a function of political climate and cultural temperament. Thus, any justification that seeks to stabilize a security policy in the name of the pursuit of certainty, finds its strength, if not its legitimacy, in uncertainty.