By "difficult," it is meant, I think, well, first of all that it is in some sense set apart from common speech, as a specialized and highly self-conscious use of language.

艰深“的意思，我认为，首先，某种程度上它是和平时谈话有所不同的，专业性极强，对语言运用自知也很高。

耶鲁公开课 - 现代诗歌课程节选

The poem seems unusually self-conscious about the role that its similes play.

这首诗看起来不同于往常地,对明喻在其中扮演的角色有所自知。

耶鲁公开课 - 弥尔顿课程节选