A question introduced by μη expects a negative answer. A question introduced by ου expects an affirmative answer. cf. BBG 31.20;
λέγει πρὸς αὐτὸν ὁ Νικόδημος· πῶς δύναται ἄνθρωπος γεννηθῆναι γέρων ὤν; μὴ δύναται εἰς τὴν κοιλίαν τῆς μητρὸς αὐτοῦ δεύτερον εἰσελθεῖν καὶ γεννηθῆναι;.
καὶ αὐτὸς ἦν ἐν τῇ πρύμνῃ ἐπὶ τὸ προσκεφάλαιον καθεύδων. καὶ ἐγείρουσιν αὐτὸν καὶ λέγουσιν αὐτῷ· διδάσκαλε, οὐ μέλει σοι ὅτι ἀπολλύμεθα;
Porter, Idioms, 277; Burton §467.