What figure of speech is I think that shall never see A poem lovely as a tree?

The figure of speech in the sentence "I think that shall never see A poem lovely as a tree" is a metaphor. A metaphor is a figure of speech that compares two unlike things without using the words "like" or "as." In this case, the poem is being compared to a tree, without explicitly saying that it is like a tree. The comparison is made based on the idea that both the poem and the tree are beautiful and inspiring in their own unique ways.