Every human being born into the world brings with them the capacity to initiate — to begin something that was not there before, to interrupt the causal chains of the natural and social world with an act that no one could have predicted. This is what Arendt means by the "miracle" of action: not a supernatural event but the quotidian miracle of free human initiative.
From Plato to Heidegger, philosophy has organised itself around the category of death — finitude, being-toward-death, the anxiety of non-being. Arendt argues that this orientation distorts political thought by directing it toward individual completion rather than collective beginning. Politics is not the domain in which individuals confront their mortality; it is the domain in which they exercise their capacity to be born again in action.
The political power of natality is linked to two capacities that Arendt identifies as essential to political life: forgiveness and promise-making. Forgiveness redeems the irreversibility of past actions; promise-making establishes islands of predictability in the sea of future contingency. Both are possible only among plural beings who share a world and can hold one another accountable.
The concept of natality appears most fully in chapter 5 of The Human Condition; Arendt also develops it in "What Is Freedom?" (1961).
