The text pages of William Blake’s manuscript An Island in the Moon end with a conspicuous diagonal pencil inscription: “a leaf is evidently missing before this one.” Although the line has been consistently regarded as a non-authorial annotation to Blake’s early satire, its influence has been decisive for interpreting the break in logic between folios 8 verso and 9 recto of the holograph. Editors have given credit to—and paraphrased—the inscription, and commentators have tacitly relied on its authority to support their readings of the work. However, the first explicit reference to the annotation dates from 2010, and it seems that thus far no inquiries have been made as to its origins. While I was working on a critical edition and translation into Spanish of An Island, the style and content of the diagonal inscription caught my attention, and several questions arose. Who might have written it, and why anonymously? Whose authoritative voice could produce such a unanimously accepted annotation? Why had the unknown hand pronounced itself about this lacuna so confidently? Was it not somewhat bold, even for nineteenth-century standards, to qualify the absence of a leaf with “evidently”? When might this annotation have been produced? Does the handwriting bear absolutely no resemblance to Blake’s? Why did Blake leave folios 10 to 15 unused? To raise suspicions further, why do the stains on folio 9 recto seem to be mirroring quite precisely the lines on folio 8 verso and, moreover, on 8 recto? And, more generally, could the satirist who had named himself Quid the Cynic in An Island be playing a prank on his readers here, as he had done before in the text? These and other questions seemed to find reasonable answers when I considered the possibility that the line might have been written by Blake himself.