Today's Quintilian: 8.6.8-10

VIII. Quaedam etiam parum speciosa dictu per hanc explicantur: "hoc faciunt nimio ne luxu obtunsior usus sit genitali arvo et sulcos oblimet inertes". In totum autem metaphora brevior est similitudo, eoque distat quod illa comparatur rei quam volumus exprimere, haec pro ipsa re dicitur. IX. Comparatio est cum dico fecisse quid hominem "ut leonem", tralatio cum dico de homine "leo est". huius vis omnis quadruplex maxime videtur: cum in rebus animalibus aliud pro alio ponitur, ut de agitatore "gubernator magna contorsit equum vi", aut [ut Livius Scipionem a Catone "adlatrari" solitum refert] inanima pro aliis generis eiusdem sumuntur, ut "classique inmittit habenas", aut pro rebus animalibus inanima: X. "ferron an fato moerus Argivom occidit"? aut contra: "sedet inscius alto accipiens sonitum saxi de vertice pastor".

8. Some things also, which are unfit to be expressed plainly, are intimated metaphorically, as,

Hoc faciunt, nimio ne luxu obtusior usus
Sit genitalis arvo, et sulcos oblimet inertes;

This they do, lest by too much indulgence
the action of the genital field should grow
too unenergetic and obstruct the inert furrows.
On the whole, the metaphor is a short comparison, differing from the comparison in this respect, that, in the one, an object is compared with the thing which we wish to illustrate. In the other, the object is put instead of the thing itself. 9. It is a comparison, when I say that a man has done something like a lion; it is a metaphor, when I say of a man that he is a lion.

Of metaphors in general there seem to be four kinds: the first, when one sort of living thing is put for another, as, in speaking of a driver of horses,

Gubernator magnâ contorsit equum vi,

The steersman turn'd his horse with mighty force;
or as Livy says that Scipio used to be barked at by Cato. 10. The second, when one inanimate thing is put for another, as,

Classique inmittit habenas,

He gives his fleet the reins.
The third, when inanimate things are put for things having life, as,

Ferro, non fato, maerus Argivum occidit,

By steel, not fate, the wall of Greece fell down;
and the fourth, when things having life are put for things inanimate,

Sedet insicius alto
Accipiens sonitum saxi de vertice pastor,

The shepherd sits amazed,
Listening the sound from the high mountain's head.