Once out of nature I shall never take My bodily form from any natural thing, (25-26)
Yup, our speaker’s not such a huge fan of the transformations that take place in the natural world. Even if the speaker’s body were to be re-crafted, it would still be mortal. That’s why art becomes a possibility, one which allows the speaker to move outside the terrifying prospect of the end of the body.
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. (13-16)
Sometimes physical movement is the only way to effect mental change. Restless in his old life, our speaker uses the trappings of an actual journey to hammer home the fact that he’s searching for spiritual transformation.