This Element addresses three questions about Kant’s guarantee thesis by examining the ‘first addendum’ of his Philosophical Sketch: how the guarantor powers interrelate, how there can be a guarantee without undermining freedom and why there is a guarantee in the first place. Kant’s conception of an interplay of human and divine rational agency encompassing nature is crucial: on moral grounds, we are warranted to believe the ‘world author’ knew that if he were to bring about the world, the ‘supreme’ good would come about too. Perpetual peace is the condition that enables the supreme good to be realized in history.