Socrates is the speaker of this passage which includes some curious assertions about sanity and madness and some adventurous folk etymologies.
Plato Phaedrus 244b-d
“If it were simply the case that insanity is evil, then this would be said truly. But, in truth, the greatest goods come to us from madness when it is given as a divine gift. For the prophet at Delphi and the priestesses at Dodona have completed many fine things for Greece both in public and private, because they were insane. But when they are in their right minds, they have done little or nothing.
And if we speak about the Sibyl and the rest—however many provide prophecy while inspired and by predicting many things for many people have improved their lives—we should clearly be spending a long time describing it all. This is also worthy of acknowledging, that the ancients who gave things names did not belief that madness was shameful or worthy of reproach. For they would not have interwoven this name—mania—with that finest art by which the future is judged.
No, instead, since it is a divine dispensation and because it is noble, they named it according to their belief. These days people call it the mantic art, callously adding a tau. When they name sane people’s investigation of the future through other means like bird omens and similar signs—since they must provide from their own perspective the mind (nous) and inquiry (historia) by means of human thought (oiêsis)—they call it oionoistic thought and make it more reverent by adding an omega as current people do. How much more complete and honorable prophecy is to augury both in name and in its action the ancients assert it is that much superior because madness is divine in origin and sanity is human.”
εἰ μὲν γὰρ ἦν ἁπλοῦν τὸ μανίαν κακὸν εἶναι, καλῶς ἂν ἐλέγετο· νῦν δὲ τὰ μέγιστα τῶν ἀγαθῶν ἡμῖν γίγνεται διὰ μανίας, θείᾳ μέντοι δόσει διδομένης. ἥ τε γὰρ δὴ ἐν Δελφοῖς προφῆτις αἵ τ᾿ ἐν Δωδώνῃ ἱέρειαι μανεῖσαι μὲν πολλὰ δὴ καὶ καλὰ ἰδίᾳ τε καὶ δημοσίᾳ τὴν Ἑλλάδα εἰργάσαντο, σωφρονοῦσαι δὲ βραχέα ἢ οὐδέν· καὶ ἐὰν δὴ λέγωμεν Σίβυλλάν τε καὶ ἄλλους, ὅσοι μαντικῇ χρώμενοι ἐνθέῳ πολλὰ δὴ πολλοῖς προλέγοντες εἰς τὸ μέλλον ὤρθωσαν, μηκύνοιμεν ἂν δῆλα παντὶ λέγοντες· τόδε μὴν ἄξιον ἐπιμαρτύρασθαι, ὅτι καὶ τῶν παλαιῶν οἱ τὰ ὀνόματα τιθέμενοι οὐκ αἰσχρὸν ἡγοῦντο οὐδὲ ὄνειδος μανίαν.οὐ γὰρ ἂν τῇ καλλίστῃ τέχνῃ, ᾗ τὸ μέλλον κρίνεται, αὐτὸ τοῦτο τοὔνομα ἐμπλέκοντες μανικὴν ἐκάλεσαν· ἀλλ᾿ ὡς καλοῦ ὄντος, ὅταν θείᾳ μοίρᾳ γίγνηται, οὕτω νομίσαντες ἔθεντο, οἱ δὲ νῦν ἀπειροκάλως τὸ ταῦ ἐπεμβάλλοντες μαντικὴν ἐκάλεσαν. ἐπεὶ καὶ τήν γε τῶν ἐμφρόνων ζήτησιν τοῦ μέλλοντος διά τε ὀρνίθων ποιουμένων καὶ τῶν ἄλλων σημείων, ἅτ᾿ ἐκ διανοίας ποριζομένων ἀνθρωπίνῃ οἰήσει νοῦν τε καὶ ἱστορίαν, οἰονοϊστικὴν ἐπωνόμασαν, ἣν νῦν οἰωνιστικὴν τῷ ω σεμνύνοντες οἱ νέοι καλοῦσιν· ὅσῳ δὴ οὖν τελεώτερον καὶ ἐντιμότερον μαντικὴ οἰωνιστικῆς, τό τε ὄνομα τοῦ ὀνόματος ἔργον τ᾿ ἔργου, τόσῳ κάλλιον μαρτυροῦσιν οἱ παλαιοὶ μανίαν σωφροσύνης τὴν ἐκ θεοῦ τῆς παρ᾿ ἀνθρώπων γιγνομένης.
The logic here might be a little bewildering, but Socrates/Plato seem to be on t something with the first etymology