De mundo (Greek: Περὶ Κόσμου), known in English as On the Universe, is the work of an unknown author who wrote under the name of Aristotle. Because of this, the author is referred to as a Pseudo-Aristotle. The date of the work is uncertain; it has been argued that it was composed before 250 BC or between 350 and 200 BC. It is found under the Bekker numbers 391–401.