Abstract: Kantian constructivists accord a constitutive, justificatory role to the issue of scope: they typically claim that first-order practical thought depends for its authority on being suitably acceptable within the right scope, or by all relevant others, and some Kantian constructivists, notably Onora O'Neill, hold that our views of the nature and criteria of practical reasoning also depend for their authority on being suitably acceptable within the right scope. The paper considers whether O'Neill-type Kantian constructivism can coherently accord this key role to the issue of scope while adhering to the universalist, ‘cosmopolitan’ commitments at its core. The paper argues that this is not so. On the one hand, it shows that O'Neill's attempt to ‘fix’ the scope of practical reasoning supposes, rather than establishes, a view of ethical standing and the scope of practical reasoning. On the other hand, the paper argues that Kantian constructivism should endorse a non-constructivist, perfectionist view of the good to determine that scope. The paper thereby supports the perfectionist conjecture that Kantian constructivism, in order to defend its universalist commitments, should take refuge in non-constructivist, perfectionist considerations, and that Kantian constructivism should therefore construe perfectionism as a partial, though uneasy, ally.