At the beginning of the poem, the speaker wants us to think that he’s pretentious. He starts talking about "new thinking" and "old thinking," like some fashion expert talking about last year’s style versus this year’s style. Then, he goes into a philosophical discussion about particular things and general ideas, without giving us any background about Plato and the ancient Greeks. (We can hear the snooty philosopher now: "You mean you didn’t know it was a reference to Plato? Humph!")
But, we get the sense that there might be more to this story when he starts sprinkling the poem with beautiful images of woodpeckers and blackberry brambles. Eventually, we get the picture: the speaker actually isn’t a pretentious guy who thinks that words don’t mean anything. In fact, he disagrees with those kinds of people. But, he’s also obviously well read and knows his stuff. He’s the kind of guy who can get away with using words like "querulous" and "numinous" without seeming like he just tries to sound smart.
In the middle of the poem (and in the middle of a line), he switches out of the pretentious act and starts telling us all this really personal stuff about his love life and childhood. One minute he’s talking about ideas and the next minute he’s like, "So, I was making love to this woman...." And we’re like, "Whoa! Where does this come from!?" This part of the poem feels like the kind of crazy late-night conversation you might have with a friend who is really honest. In a strange way, the abrupt transition from philosophy to sex and back again makes perfect sense, in the way that even the wildest late-night conversations always seem to – at least, at the time.