Plutarch's Platonic questions (9.1)

Since Plato in his Commonwealth, discoursing of the faculties of the soul, has very well compared the symphony of reason and of the irascible and the concupiscent faculties to the harmony of the middle, lowest, and highest chord,[*] some men may properly ask this question:—

DID PLATO PLACE THE RATIONAL OR THE IRASCIBLE FACULTY IN THE MIDDLE? FOR HE IS NOT CLEAR IN THE POINT.

Indeed, according to the natural order of the parts, the place of the irascible faculty must be in the middle, and of the rational in the highest, which the Greeks call hypate. For they of old called the chief and supreme ὕπατος. So Xenocrates calls Jove, in respect of immutable things, ὕπατος (or highest), in respect of sublunary things

νέατος (or lowest.) And long before him, Homer calls the chief God ὕπατος κρειότων, Highest of Rulers. And Nature has of due given the highest place to what is most excellent, having placed reason as a steersman in the head, and the concupiscent faculty at a distance, last of all and lowest. And the lowest place they call νεάτη, as the names of the dead, νέρτεροι and ἔνεροι, do show. And some say, that the south wind, inasmuch as it blows from a low and obscure place, is called νότος. Now since the concupiscent faculty stands in the same opposition to reason in which the lowest stands to the highest and the last to the first, it is not possible for the reason to be uppermost and first, and yet for any other part to be the one called ὕπατος (or highest). For they that ascribe the power of the middle to it, as the ruling power, are ignorant how they deprive it of a higher power, namely, of the highest, which is competible neither to the irascible nor to the concupiscent faculty; since it is the nature of them both to be governed by and obsequious to reason, and the nature of neither of them to govern and lead it. And the most natural place of the irascible faculty seems to be in the middle of the other two. For it is the nature of reason to govern, and of the irascible faculty both to govern and be governed, since it is obsequious to reason, and commands the concupiscent faculty when this is disobedient to reason. And as in letters the semi-vowels are middling between mutes and vowels, having something more than those and less than these; so in the soul of man, the irascible faculty is not purely passive, but hath often an imagination of good mixed with the irrational appetite of revenge. Plato himself, after he had compared the soul to a pair of horses and a charioteer, likened (as every one knows) the rational faculty to the charioteer, and the concupiscent to one of the horses, which was resty and unmanageable altogether, bristly about the ears, deaf and disobedient both to whip and spur; and the irascible he makes for the most part very obsequious to the bridle of reason, and assistant to it. As therefore in a chariot, the middling one in virtue and power is not the charioteer, but that one of the horses which is worse than his guider and yet better than his fellow; so in the soul, Plato gives the middle place not to the principal part, but to that faculty which has less of reason than the principal part and more than the third. This order also observes the analogy of the symphonies, i.e. the relation of the irascible to the rational (which is placed as hypate) forming the diatessaron (or fourth), that of the irascible to the concupiscent (or nete) forming the diapente (or fifth), and that of the rational to the concupiscent (as hypate to nete) forming an octave or diapason. But should you place the rational in the middle, you would make the irascible farther from the concupiscent; though some of the philosophers have taken the irascible and the concupiscent faculty for the selfsame, by reason of their likeness.