In this article, I examine a Latin paraphrase of Aristotle's De caelo known as the Liber celi et mundi. The text was translated from Arabic in the third quarter of the twelfth century, and thus pre-dates all four Latin translations of De caelo in the twelfth and thirteenth centuries. It was probably written by the ninth century Arab, Hunayn ibn Ishaq. I show the weakness of a previous theory that the Liber celi et mundi derives indirectly from Themistius's paraphrase of De caelo. The text was translated into Latin by Dominicus Gundissalinus and his Jewish colleague, Johannes Hispanus. From c.1250, it was mis-attributed to Avicenna, and there is evidence that it had earlier been attributed to Aristotle by certain English writers. I consider the function of the Liber cell el mundi within the corpus of early Aristotelian translations, and the date of its expulsion from the corpus.