If politics is understood as a foundational and open-ended activity, a general problem that arises from such a framing concerns the question of how to sustain the possibility of continuous openings without converting action into permanence and closure. In this article, we approach this problematic by treating Hannah Arendt as an exemplary figure in the current of political thought that emphasizes the indeterminate nature of action. We focus more specifically on how Arendt addressed the question of sustaining action by exploring (...) the role of forgiveness, promises, divided power, and principles of action in her thought. While we show that the task of sustaining the indeterminacy of action partly remains an unresolvable paradox for Arendt, a general force underpinning the thought of Giorgio Agamben sheds new light on this paradox. Rather than affirming radical openness and pure _praxis_ as such, Agamben’s work helps accentuate the need of a certain contamination of _praxis_ by its own limit or apparent opposite. Importantly, this allows us to interpret Arendt and the paradoxical task of bestowing some degree of permanence to the open horizon of political action in a decidedly positive light: that _praxis_ implies a moment of impurity is the very force that sustains it without sacrificing its indeterminate nature. (shrink)