Could a work of art actually authored by one artist have been authored, instead, by another? This is the question of the necessity of authorship. After distinguishing this question from another, regarding individuation, with which it is often confused, this paper offers an argument that authorship is indeed a necessary feature of most artworks. The argument proceeds from ‘independence principles’, which govern the processes by which artworks are produced. Independence principles are motivated, in turn, by metaphysical reflections on what it takes to prevent an artist from producing a particular work of art.