The paper offers a rights-based, Kantian interpretation of the idea/expression dichotomy in the law of copyright. It demonstrates that the idea/expression dichotomy normatively structures the relation between the parties to a copyright action in terms of their equal rights to authorship. To the extent that the defendant has not copied the plaintiff’s expression but has instead expressed an idea anew, the defendant has exercised her own authorship. The limits of the plaintiff’s right (i.e. the law’s refusal to copyright ideas) are therefore the contours of a public domain that, as a matter of equality, the plaintiff himself must be held to recognize. The public domain is not externally imposed upon but internally constitutive of authorial right. Thus the paper shows that a Kantian understanding of the fundamentals of copyright questions the perceived opposition between authorial right and public domain that informs much of contemporary copyright discourse. In so doing, the paper establishes the largely neglected possibility of a rights-based defense of the public domain.