Poetry is the communication through words of certain experiences that can be communicated in no other way.
So it is in poetry. All we ask is that the mood recorded shall impress us as having been of the kind that exhausts the imaginative capacity if it fails to do this the failure will announce itself either in prose or in insignificant verse.
There can be no proof that Blake's lyric is composed of the best words in the best order only a conviction, accepted by our knowledge and judgment, that it is so.