Protagoras, Fr. F2 (Didymus the Blind, On the Psalms 222.20-25)

“I see the moon but somone else does not. It is unclear whether or not it exists.”

οἷον ὁρῶι τὴν σελήνην, ἄλλος δὲ οὐκ ὁρᾶι. ἄδηλον εἰ ἔστιν ἢ οὐκ ἔστιν.