The present publication presents the edition of an Irish treatise on the universe, composed in the ninth or tenth century. This work, which purportedly records a revelation of the mysteries of the cosmos uttered in angelic language by the soul of the apostle Philip, is characterized by the vividness of its imagery and the rich diversity of its content. Besides providing the most conservative version of the text, preserved in the Book of Lismore, the book supplies on facing pages a full critical edition of the second recension, found in four further manuscripts. Both versions are accompanied by translation. An introduction traces the text’s transmission from the time of its composition down to the final flowering of the Irish scribal tradition in the eighteenth and nineteenth centuries; undertakes to identify its sources in earlier apocalyptic and cosmological literature; and subjects it to an in-depth linguistic analysis in order to place the question of its date in a clearer light. Individual aspects of the work’s content are discussed in an extended commentary, while matters of specifically philological interest are covered in a section of textual notes.