This paper looks at two different views of the universality of the constraint set in Optimality Theory. The first view is that the universality is a statement of mental reality of speakers of human languages, and so is a real, objective fact of the world. The second view is that the constraint set is a convention, like the phonetic alphabet, which reflects some functionally motivated categories, but is at heart arbitrary. I examine seven arguments for one view or the other, and conclude that

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... arguments for the objectivist position are weak, while it is hard deny the usefulness of a conventional constraint set.