The debate over the public domain in intellectual property lawsuits shape, its importance, its contents has been largely policy-driven, with varying positions shaped by the philosophical, economic and constitutional perspectives of the participants. This article examines the nature of the public domain from a different angle, asking not what an ideal public domain would look like, but asking instead whether some elements of the public domain are actually fixed in place by constitutional principles so that they cannot be commodified no matter how persuasive the normative justifications might be for doing so. Intimations of this can be found in intellectual property cases—in particular in Harper & Row and in Eldred—but with little explication. In First Amendment case law, too, there have been recurrent but undeveloped references to content that can no longer be subjected to private or public regulation because it has entered the public domain. The purpose of this Article is to ask how seriously to take these allusions to a mandatory public domain, composed of content that permanently available to all to use as they wish. The ultimate conclusion is, very seriously. Before reaching that answer, an account of why it makes sense to hypothesize a constitutionally mandated information commons is provided and the various sources of legal doctrine that support, and in some instances undercut, the case that one exits are sorted through. Finally, the Article sketches in broad strokes what this postulated mandatory public domain must contain.