We talked about it late last night and in the voice of my friend, there was a thin wire of grief, a tone almost querulous.
The speaker just scoops us up and parachutes us down into his late-night conversation with one of his friends.
They talked "last night," whenever that was, which means that the subject is fresh on his mind. It was probably one of those conversations where it’s so late and you’re so tired that you think everything you say sounds really smart and important.
It’s a "serious" conversation, probably a friendly argument. We don’t know who took which side: if the friend was on Plato’s side, or if it was the speaker. But, we know that the friend, at least, gets frustrated. His voice expresses "grief," as if he mourns something, and his tone is "querulous," which means he complains a lot.
We’re still on the subject of philosophy here, but, even so, the speaker sneakily keeps trying to turn language into everyday things.
The image of the "thin wire of grief" is, we think, one of the most beautiful in the poem. In our mind, we can see the particular thing – a metal wire – combine with this totally abstract idea of grief.
After a while I understood that, talking this way, everything dissolves: justice, pine, hair, woman, you and I.
It isn’t just the "friend" who gets frustrated with the overly philosophical conversation. After a while, the speaker realizes that all this talk about how words are reminders of some lost perfection actually causes imperfect, everyday things to become lost as well.
The everyday world "dissolves," or disappears, in the face of this discussion about what’s real and what’s not. All these things we talk about, from "justice" to "pine" trees to a "woman," all become fancy ideas instead of particular things in the world.
For the speaker, this isn’t a good outcome. He’s a poet, not a philosopher, and he wants words to connect to the everyday world.
Basically, he’s saying, "There’s no point in thinking too much about how language works – or you’ll realize that it doesn’t work at all!"