Ille Ego Qui Quondam

Those widely suppressed first-four-lines-to-the-Aeneid:

Ille ego quondam gracili modulatus avena
carmen at egressus silvis vicina coegi
ut quamvis avido parerent arva colono
gratum opus agricolis; ad nunc horrentia Martis.

Spenser was fond of this little passage; he structured his own career upon it, and brought the lines themselves into both the Shepheard's Calendar:

Indeede the Romish Tityrus, I heare,
Through his Mecænas left his Oaten reede,
Whereon he earst had taught his flocks to feede,
And laboured lands to yield the timely eare,
And eft did sing of warres and deadly drede,
So as the heauens did quake his verse to here.

But ah Mecænas is yclad in claye,
And great Augustus long ygoe is dead:
And all the worthies liggen wrapt in leade,
That matter made for Poets on to play:
For euer, who in derring doe were dreade,
The loftie verse of hem was loued aye. [IX 'October']

And the opening of the Faerie Queene:

Lo I the man, whose Muse whilome did maske,
As time her taught, in lowly Shepheards weeds,
Am now enforst a far unfitter taske,
For trumpets sterne to chaunge mine Oaten reeds,
And sing of Knights and Ladies^ gentle deeds;
Whose prayses having slept in silence long,
Me, all too meane, the sacred Muse areeds
To blazon broade emongst her learned throng:
Fierce warres and faithfull loves shall moralize my song.

They were all at it, mind. Here's how Lope de Vega opens his epic Jerusalén conquistada (1609):

Yo que canté oara la tierna vuestra
los amores de Angélica y Medoro
en otra edad, con otra voz mas diestra;
de vuestro sol el vivo rayo adoro;
en tanto, pues, que a la marcial palestra
la fama os llama en al metal sonoro,
oid, Felipe, las heroicas sumas
de Espana triunfos, de la fama plumas.

[I who in another time sang for your tender years with a sweeter voice about the love between Angelica and Medoro, now I come to worship the bright rays of your sun. Whilst Fame now summons you, Philip, to the arena of war, listen now to the supreme heroic achievements, the triumphs of Span, the famous writing.]

David Scott Wilson-Okamura's Virgil in the Renaissance (2010) explores the ins-and-outs of this in fascinating detail. He suggests the three part trajectory entails various other logics, and quotes John of Garland's 13th-century 'Wheel of Vergil':

Lowly (humilis) style: Shepherd at ease (Tityrus, Melibeous); animal -- sheep; tool -- crook; location -- pasture; tree -- beech.

Middle (mediocris style: Farmer (Triptolemus, Coelius); animal -- cow; tool -- plough; location -- field; tree -- fruit (apple, pear)

Weighty (grauis) style: Soldier, Prince, King (Hector, Ajax, Achilles); animal -- horse; tool -- sword; location -- city, camp, battlefield; tree -- laurel, cedar. [Wilson-Okamura, 91]

What's really going on here, though, is made clearer by Milton's characteristically complex reworking of this device:

Sing, Heavenly Muse, that, on the secret top
Of Oreb, or of Sinai, didst inspire
That shepherd who first taught the chosen seed
In the beginning how the heavens and earth
Rose out of Chaos: or, if Sion hill
Delight thee more, and Siloa's brook that flowed
Fast by the oracle of God, I thence
Invoke thy aid to my adventurous song,
That with no middle flight intends to soar
Above th' Aonian mount, while it pursues
Things unattempted yet in prose or rhyme.
And chiefly thou, O Spirit, that dost prefer
Before all temples th' upright heart and pure,
Instruct me, for thou know'st; thou from the first
Wast present, and, with mighty wings outspread,
Dove-like sat'st brooding on the vast Abyss,
And mad'st it pregnant: what in me is dark
Illumine, what is low raise and support;
That, to the height of this great argument,
I may assert Eternal Providence,
And justify the ways of God to men.

This is tripartite, too: shepherd, brook, the pure spirit. The shepherd is Moses -- who was, indeed, a shepherd. Indeed he was working as a shepherd when he encountered the burning bush, that puzzling rebus of God. My take on the burning bush is that it embodies the essential mystery of God's creation as seasonal, the same deep force that drives pastroal and bucolic poetry through its green fuse: the world of vegetation on which we depend for life itself (first hand as food, second hand as food for our animals, and -- as we now comprehend -- zeroth hand for the very oxygen that we breathe) ... that world dies every winter, and yet is reborn every Spring. It is continually consumed, as by fire (not least by us) but despite this it is continually alive. (It's not accidental that in the Qu'ran, Moses is taught by Khidr, the Green Man beloved of God, a figure of youthful vigour and handsomeness although with a long white beard and the wisdom of old age ... creation embodied as simultaneously dying and coming-to-life, John Barleycorn, the Green Knight). The Moses narrative contains all three: Moses himself as pastor, the 'cow'-based bucolic element of the Golden Calf which inflame Mosaic ire, and the mountain into which Moses ascends to encounter the pure spirit.