It's false to say that, rather than someone who loves you madly and is good to go, you should take a disinterested lover who is sane and rational. That would be well said if all madness were bad. But it is through madness that our greatest blessings come to us, by which of course I mean the madness that is the gift of the gods. [Firstly,] in public as in private matters, the ravings of the oracle at Delphi have done Greece a lot of good, and so have the holy women who prophesy at Dodona, but little to no good when these same women were in their right minds. And if we were to speak of the Sibyl, and all others whose divinely-inspired pronunciations have corrected so many people's courses toward the future, then our discourse would obviously run on long.
But it is worth giving evidence for the beliefs of the ancient name-givers, according to whom madness was no cause for rebuke or shame. Otherwise, they would not have called our noblest prognostic arts by a name that implicates them in mania. But in their conviction that divinely-awarded madness is a blessing, they designated these arts as manic; it's only now that the "mantic arts" are spoken of with an inserted letter t, which is an insipid vulgarism. [By contrast,] when they assigned a name to those forms of research into the future performed by the non-mad, through studious contemplation of birds and other omens, they called them oionoïstikē, since these techniques endow mortal oiēsis (opinion) with nous (intellect) and historia (fruits of inquiry). Nowadays, by way of affecting a more sententious tone, people lengthen the second o and pronounce it as oiōnoïstikē. The upshot of all that is this: To the same degree that mantic arts are more perfect and honorable than augury—in name as they are in deed—the superiority of divine madness to mortal reason is attested by the ancients.
It also happens, in the event of ailments and grievous harms stemming from accursed deeds of long ago [e.g.], that madness intervenes to communicate a divinely-inspired message to those in need, and through resort to prayer and ministration to the gods it ferrets out their means of deliverance, hitting thus upon purifications and sacred rituals and bringing wellness once and for all to the sufferer touched with madness. Madness finds release for people in the grip of present evils, provided that they rave in the right way.
Thirdly, there is possession by the Muses. This madness takes hold of pure and tender souls and stirs them to song and other verse forms in a Bacchic frenzy. Thus arranged by the Muses' madness, countless feats of the heroic past are made teachable to hearers of the latter day. Anyone who shows up at the gates of poetry without it, presuming to become a worthy poet through craft alone, is destined for oblivion when the poetry of the stark and raving blows away that of the merely sane.
Plato, Phaedrus 244a-245a