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Thomas Scanlon provides a theory of why we ought to keep our promises according to which the wrong of breaking a promise is a moral wrong that does not depend on any social practice. Instead a promise provides a recipient with assurance and the value of assurance establishes a moral obligation to keep our promises. However, it is often charged that theories like Scanlon’s are untenable because they are subject to a vicious circularity. I address some recent critics of Scanlon’s theory, all of whom maintain that his account does not adequately show how a promise provides assurance and therefore does not overcome the charge of circularity in explaining why we are obligated to keep our promises. I revise Scanlon’s theory and show how a promise can provide a recipient with assurance, demonstrating that Scanlon’s account is a tenable theory of why we have an obligation to keep our promises.