Full Idea
What is over our heads ...or what is below any point which we think of ...will never appear to us as being at the same time and in the same respect both up and down. For it is impossible to conceive of this.
Gist of Idea
Above and below us will never appear to be the same, because it is inconceivable
Source
Epicurus (Letter to Herodotus [c.293 BCE], 60)
Book Reference
Epicurus: 'The Epicurus Reader', ed/tr. Inwood,B. /Gerson,L. [Hackett 1994], p.12
A Reaction
Note that he says it will 'never appear to us' as both - not that it absolutely cannot be both. Both Aristotle and Epicurus are much more focused on how our humanity shapes our metaphysics than the modern pure metaphysicians are.