<p>This article develops a diagnostic account of self-cancellation, a distinctive practical failure in which an agent relies on a rule-governed practice while handling a constitutive commitment so that it cannot do its settling work. The defect is not best described as a false belief, a broken rule, or a bad outcome. It is a failure of participation, in that the performance no longer counts, in the relevant sense, as planning, inferring, promising, or legal reasoning. The paper reinterprets Kant’s question What if everyone did that? as a non-moral diagnostic use of the Formula of Universal Law. Universalization is used to test whether a way of proceeding can remain intelligible once it is treated as generally available. On this basis, the article replaces accounts in terms of volitional self-contradiction with a commitment-based diagnosis of practice failure. This diagnosis is then applied to planning and shared agency, Carroll’s regress, and two cases from legal theory, namely Kelsen’s Pure Theory of Law and the role of background standards in legal reasoning.</p>