Giovanni Boccaccio, on allegory and poetry in The Genealogies of the Pagan Gods (Genealogie Deorum), Book 14.

From Genealogie Deorum XIV, 7:

This poetry, which ignorant triflers cast aside, is a sort of fervid and exquisite invention, with fervid expression, in speech or writing, of that which the mind has invented. It proceeds from the bosom of God, and few, I find, are the souls in whom this gift is born; indeed so wonderful a gift it is that true poets have always been the rarest of men. This fervor of poesy is sublime in its effects: it impels the soul to a longing for utterance; it brings forth strange and unheard-of creations of the mind; it arranges these meditations in a fixed order, adorns the whole composition with unusual interweaving of words and thoughts; and thus it veils truth in a fair and fitting garment of fiction. Further, if in any case the invention so requires, it can arm kings, marshal them for war, launch whole fleets from their docks, nay, counterfeit sky, land, sea, adorn young maidens with flowery garlands, portray human character in its various phases, awake the idle, stimulate the dull, restrain the rash, subdue the criminal, and distinguish excellent men with their proper meed of praise: these, and many other such, are the effects of poetry. Yet if any man who has received the gift of poetic fervor shall imperfectly fulfil its function here described, he is not, in my opinion, a laudable poet. For, however deeply the poetic impulse stirs the mind to which it is granted, it very rarely accomplishes anything commendable if the instruments by which its concepts are to be wrought out are wanting -- I mean, for example, the precepts of grammar and rhetoric, an abundant knowledge of which is opportune. I grant that many a man already writes his mother tongue admirably, and indeed has performed each of the various duties of poetry as such; yet over and above this, it is necessary to know at least the principles of the other Liberal Arts, both moral and natural, to possess a strong and abundant vocabulary, to behold the monuments and relics of the Ancients, to have in one's memory the histories of the nations, and to be familiar with the geography of various lands, of seas, rivers and mountains.

Furthermore, places of retirement, the lovely handiwork of Nature Herself, are favorable to poetry, as well as peace of mind and desire for worldly glory; the ardent period of life also has very often been of great advantage. If these conditions fail, the power of creative genius frequently grows dull and sluggish.

Now since nothing proceeds from this poetic fervor, which sharpens and illumines the powers of the mind, except what is wrought out by art, poetry is generally called an art. Indeed the word poetry has not the origin that many carelessly suppose, namely poio, pois, which is but Latin fingo, fingis; rather it is derived from a very ancient Greek word poetes, which means in Latin exquisite discourse (exquisita locutio). For the first men who, thus inspired, began to employ an exquisite style of speech, such, for example, as song in an age hitherto unpolished, to render this unheard-of discourse sonorous to their hearers, let it fall in measured periods; and lest by its brevity it fail to please, or on the other hand, become prolix and tedious, they applied to it the standard of fixed rules, and restrained it within a definite number of feet and syllables. Now the product of this studied method of speech they no longer called by the more general term poesy, but poem. Thus as I said above, the name of the art, as well as its artificial product, is derived from its effect.

Now though I allege that this science of poetry has ever streamed forth from the bosom of God upon souls while even yet in their tenderest years, these enlightened cavilers will perhaps say that they cannot trust my words. To any fair-minded man the fact is valid enough from its constant recurrence. Rut for these dullards I must cite witnesses to it. If, then, they will read what Cicero, a philosopher rather than a poet, says in his oration delivered before the senate in behalf of Aulus Licinius Archias, perhaps they will come more easily to believe me. He says: "And yet we have it on the highest and most learned authority, that while other arts are matters of science and formula and technique, poetry depends solely upon an inborn faculty, is evoked by a purely mental activity, and is infused with a strange supernal inspiration."

But not to protract this argument, it is now sufficiently clear to reverent men, that poetry is a practical art, springing from God's bosom and deriving its name from its effect, and that it has to do with many high and noble matters that constantly occupy even those who deny its existence. If my opponentss ask when and in what circumstances, tile answer is plain: the poets would declare with their own lips under whose help and guidance they compose their inventions when, for example, they raise flights of symbolic steps to heaven, or make thick-branching trees spring aloft to the very stars, or go winding about mountains to their summits. Haply, to disparage this art of poetry now unrecognized by them, these men will say that it is rhetoric which the poets employ. Indeed, I will not deny it in part, for rhetoric has its own inventions. Yet, in truth, among the disguises of fiction rhetoric has no part, for whatever is composed as under a veil, and thus exquisitely wrought, is poetry and poetry alone.

From Genealogie Deorum XIV, 14:

They would cease to wonder that the poets call Jove, now god of heaven, now lightning, now an eagle, or a man, or whatever, if they had only reminded themselves that Holy Write itself from time to time represents the one true God as sun, fire, lion, serpent, lamb, worm, or even a stone. Likewise our most venerable mother the Church is prefigured in the sacred books, sometimes as a woman clothed with the sun, or arrayed in varied garb, sometimes as a chariot or a ship, or an ark, a house, a temple, and the like. No less is this true of the Virgin Mother, or of the Great Enemy of mankind, as I remember to have read, time and again. I can say the same also of the multiplicity of sacred epithets; these applied to God alone are indeed innumerable at present, as are those of the Virgin Mary and the Church. Such forms and epithets are not devoid of mystic meaning; no more are those employed by poets.