The intellect that W B Yeats values clearly has an occult and spiritual, rather than a material, basis. The golden bough is recognisable both as the mystical token that allows Aeneas to enter the underworld in book six of the Aeneid and the title of a famous book by Sir James Fraser on magic and comparative religion. Yeats may have had in mind the 6th century mosaic of the Byzantine Emperor Justinian I from the Basilica of San Vitale in Ravenna: see it in the illustrated blog post here.
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.
An aged man is but a paltry thing,
A tattered coat upon a stick, unless
Soul clap its hands and sing, and louder sing
For every tatter in its mortal dress;
Nor is there singing school but studying
Monuments of its own magnificence,
And therefore I have sailed the seas and come
To the holy city of Byzantium.
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.
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;
Or perch upon a golden bough to sing
To lords and ladies of Byzantium
Of what is past, or passing, or to come.