That is no country for old men. The young
In one another's arms, birds in the trees,
—Those dying generations—at their song,
The salmon-falls, the mackerel-crowded seas,
Fish, flesh, or fowl, commend all summer long
Whatever is begotten, born, and dies.
Caught in that sensual music all neglect
Monuments of unageing intellect.

This passage comprises the poem’s full opening stanza (lines 1–6). The famous opening line gestures back to the country the speaker has already left, signaling that he’s giving a retrospective account of why he decided to leave. This decision is partly based on the culture of his native land, which seems to be preoccupied with youth as well as the pleasures of material existence. That is, the people of his society spend their time celebrating “whatever is begotten, born, and dies.” The main problem for the speaker seems to be that, by getting “caught in that sensual magic,” the people around him tend to neglect “monuments of unageing intellect.” That is, their focus on passing pleasures causes them to ignore the eternal truths enshrined in cultural “monuments” such as religious traditions and works of art. In this way, the opening stanza introduces a central tension between mortality and immortality—that is, between the aging quality associated with material existence and the “unageing” quality associated with spiritual and aesthetic truth. To put the matter simply, the aging speaker is tired of worldly concerns and is turning his mind to spiritual matters.

O sages standing in God's holy fire
As in the gold mosaic of a wall,
Come from the holy fire, perne in a gyre,
And be the singing-masters of my soul.
Consume my heart away; sick with desire
And fastened to a dying animal
It knows not what it is; and gather me
Into the artifice of eternity.

This passage comprises the poem’s third stanza (lines 17–24). This is a key section in the poem, since it’s where the speaker first articulates a vision for what his spiritual transformation will entail now that he’s arrived in Byzantium. The speaker prays to a group of “sages,” asking them to be the “singing-masters” of his soul and usher him into “the artifice of eternity.” The vision here is one of spiritual salvation, in which the sages will help liberate the speaker’s soul from his body, spinning him like a spool (or “perne”) in a whirling “gyre”—a word Yeats often used to reference fate. The “sages” referenced here allude to a story about the Hagia Sophia prior to its conversion into an Islamic mosque. According to legend, when the Ottomans invaded Constantinople in 1453, the priests are said to have disappeared into the church walls to await the return of Christendom. The speaker uses a simile to make these invisible sages visible. No longer hidden inside the walls, they appear “in the gold mosaic of a wall.” Here, Yeats is making an allusion to the mosaic of the holy martyrs in the Basilica di Sant’Apollinare Nuovo in Ravenna, Italy, which he had visited in 1907.

Once out of nature I shall never take
My bodily form from any natural thing,
But such a form as Grecian goldsmiths make
Of hammered gold and gold enamelling
To keep a drowsy Emperor awake

Lines 25–29 open the poem’s fourth and final stanza. Here, the speaker turns from the religious notion of spiritual salvation to envision a very different form of transformation. Specifically, the speaker imagines himself being transformed into an art object. Once he’s “out of nature” and hence no longer trapped in his degrading body, the speaker vows never again to return to an organic form. Instead, he sees himself taking the form of an artificial bird made of gold in the style of Grecian goldsmiths. It isn’t immediately obvious that the object “of hammered and gold enamelled” is in fact a bird. Here, it’s helpful to recognize that Yeats is referencing a fairy tale by Hans Christian Andersen, in which the emperor of China keeps an artificial nightingale in his garden. Similarly, the speaker imagines himself becoming a bird that will live in the Byzantine emperor’s garden and keep him awake with his mechanical birdsong. If the imagery in stanza 3 reflected the speaker’s desire to be gathered into the eternity of the Christian faith, the imagery in the final stanza reflects his desire literally to become a work of art. In other words, he aims to achieve immortality through the quasi-spiritual truth enshrined aesthetic form.