An Island in the Moon is the name generally assigned to an untitled, unfinished prose satire by William Blake, written in late 1784. Containing early versions of three poems later included in Songs of Innocence (1789) and satirising the "contrived and empty productions of the contemporary culture", An Island demonstrates Blake's increasing dissatisfaction with convention and his developing interest in prophetic modes of expression. Referred to by William Butler Yeats and E. J. Ellis as "Blake's first true symbolic book," it also includes a partial description of Blake's soon-to-be-realised method of illuminated printing. The piece was unpublished during Blake's lifetime, and survives only in a single manuscript copy, residing in the Fitzwilliam Museum, in the University of Cambridge.

The overriding theory as to the main impetus behind An Island is that it allegorises Blake's rejection of the bluestocking society of Harriet Mathew, who, along with her husband, Reverend Anthony Stephen Mathew organised 'poetical evenings' to which came many of Blake's friends (such as John Flaxman, Thomas Stothard and Joseph Johnson) and, on at least one occasion, Blake himself. The Mathews had been behind the publication in 1783 of Blake's first collection of poetry, Poetical Sketches, but by 1784, Blake had supposedly grown weary of their company and the social circles in which they moved, and chose to distance himself from them. This theory can be traced back to an 1828 'Biographical Sketch' of Blake by his friend in later life, the painter J. T. Smith, published in the second volume of Smith's biography of Joseph Nollekens, Nollekens and his times. Smith's references to the Mathew family's association with Blake were taken up and elaborated upon by Blake's first biographer, Alexander Gilchrist, in his 1863 biography Life of William Blake, Pictor Ignotus., and from that point forth, the prevailing belief as to the primary background of An Island is that it dramatises Blake's disassociation from the social circles in which he found himself.

Critical work in the second half of the twentieth century, however, has often challenged the assumption that An Island originated in Blake's rejection of a specific social circle. Foremost amongst such work is that of David V. Erdman, who suggests instead that the main background to the An Island is Blake's belief in his own imminent financial success. In early 1784, Blake opened a print shop at No. 27 Broad Street with James Parker, alongside whom he had served as an apprentice to the engraver James Basire during the 1770s. At the time, engraving was becoming an extremely lucrative trade, accruing both wealth and respectability for many of its practitioners, and Erdman believes that the increasing prosperity for engravers in the early 1780s represents the most important background to An Island, arguing that the confidence which Blake and Parker must have felt informs the content more so than any sense of social rejection; "the kind of envy that breeds satire is that of the artist and artisan who is anticipating the taste of success and is especially perceptive of the element of opportunism." Erdman also sees as important the fact that the character based on Blake, Quid the Cynic, partially outlines a new method of printing, not unlike Blake's own, as yet unrealised, illuminated printing. Quid argues that he will use this new method of printing to outdo the best-known and most successful of artists and writers, such as Joshua Reynolds, William Woollett, Homer, John Milton and William Shakespeare. Behind this claim, argues Erdman, "lies the vision of a man [...] who begins to see a way to replace the division of labour with the harmony of One Man, to renew and join together the arts of poetry and painting without going outside his own shop and his own head." As such, Erdman contends that the primary background factor for An Island is the sense of anticipation and exuberance on the part of Blake, expectation for his new business venture and excitement regarding his new method of printing; An Island was thus borne from anticipation.

Nevertheless, writing in 2003, Nick Rawlinson, who also disagrees with the 'rejection theory', points out that "the general critical consensus is that the eleven surviving chapters of this unpublished manuscript form little more than Blake's whimsical attempt to satirise his friends, neighbours and fellow attendees of 27 Rathbone Place, the intellectual salon of the Reverend and Mrs AS Mathew; a kind of pleasing cartoon wallpaper on which he couldn't resist scrawling a few grotesque caricatures of his favourite scientific and philosophic bugbears."

Due to the nature of the revisions in the only existing manuscript copy of An Island, it is generally agreed amongst scholars that the manuscript is not the original, but was a copy made by Blake. Blake seems to have worked on the eighteen-page MS over eight sittings, as there are eight different types of ink used throughout. The manuscript also contains many handwritten corrections in Blake's handwriting.

Also of interest is that on the last page of the MS are found numerous small pencil drawings of horses, lambs, lions and two human profiles. Additionally, the word "Numeration" has been written in the centre of the page, the word "Lamb" in tiny script between the two human profiles (partly obscured by a large "N"), and two signatures of Blake himself at the top of the page. Also present are various random letters (especially the letter "N") which may be examples of Blake's attempts to master mirror writing, a skill which was necessary for his work as an engraver. However, it is thought that at least some of the sketches and lettering on this page could have been by Blake's brother, Robert; "the awkwardness and redundancy of some of the work, the bare geometry of the head of the large lion in the lower pair of animals in the upper left quadrant of the page, and the heavy overdrawing on some of the other animals are among the features that may possibly reflect Robert's attempts to draw subjects that had been set as exercises for him by older brother William, and, in some instances, corrected by one of them."

Although the MS contains no date, due to certain topical references, An Island is generally believed to have been composed in late 1784. For example, there is a reference to the Great Balloon Ascension of September 15, when England's first airship rose from the Artillery Ground in Finsbury, watched by 220,000 Londoners. Other topical allusions include references to a performing monkey called Mr. Jacko, who appeared at Astley's Amphitheatre in Lambeth in July, a performing pig called Toby the Learned Pig, a Handel festival in Westminster Abbey in August, lectures on phlogiston, exhibitions of the microscope, and the Golden Square parties of Chevalier d'Eon.

Especially important in dating the text is Miss Gittipin's reference to Miss Filligree; "theres Miss Filligree work she goes out in her coaches & her footman & her maids & Stormonts & Balloon hats & a pair of Gloves every day & the sorrows of Werter & Robinsons & the Queen of Frances Puss colour." Stormonts were a type of hat popular in the early 1780s but had fallen out of fashion by 1784. Balloon bonnets (linen cases stuffed with hair) had become extremely popular by late 1784, as had Robinson hats and gowns (named after the poet and actress Mary Robinson). The mention of Werter is also a reference to a hat, not the 1774 novel. The balloon bonnet and the Robinson hat were in vogue from late 1783 to late 1784, when they overlapped slightly with the increasing popularity of the Werter. By mid-1785, however, all three had fallen out of fashion. This serves to situate An Island in late 1783-late 1784, and taken in tandem with the topical references, seems to confirm a date of the latter half of 1784.