Plutarch's Platonic questions (9.2)

But it may be ridiculous to describe the first, middle, and last by their place; since we see hypate highest in the harp, lowest in the pipe; and wheresoever you place the mese in the harp, provided it is tunable, it sounds more acute than hypate, and more grave than nete. Nor does the eye possess the same place in all animals; but whereever it is placed, it is natural for it to see. So a pedagogue, though he goes not foremost but follows behind, is said to lead (ἄγειν), as the general of the Trojan army,

Now in the front, now in the rear was seen,

And kept command;

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but wherever he was, he was first and chief in power. So the faculties of the soul are not to be ranged by mere force in order of place or name, but according to their power and analogy. For that in the body of man reason is in the highest place, is accidental. But it holds the chief and highest power, as mese to hypate, in respect of the concupiscent; as mese to nete, in respect of the irascible; insomuch as it depresses and heightens,—and in fine makes a harmony,—by abating what is too much and by not suffering them to flatten and grow dull. For what is moderate and symmetrous is defined by mediocrity. Still more is it the object of the rational faculty to reduce the passions to moderation, which is called sacred, as effecting a harmony of the extremes with reason, and through reason with each other. For in chariots the best of the beasts is not in the middle; nor is the skill of driving to be placed as an extreme, but it is a mediocrity between the inequality of the swiftness and the slowness of the horses. So the force of reason takes up the passions irrationally moved, and reducing them to measure, constitutes a mediocrity betwixt too much and too little.