Scanning ahead, we can see that there are actually five sections of this book with this same title, "A Memorable Fancy." We've used numbers to make it clear just which one of those we're talking about.
Did Blake just really love that title? Or was he just super-lazy when it came to naming these sections?
As it turns out, it's neither. These sections are joking references to the "Memorable Relations" entries in another of Emmanuel Swedenborg's books, Apocalypse Revealed. But for now just know that this is Blake taking another dig at the dude.
In this first fancy, our speaker tells us all about a trip to Hell—sounds like a blast.
In fact, our speaker did have a good time. He enjoyed experiencing the local "Genius" (which angels really hate) and he got some souvenirs as well: some proverbs.
He tells us that the sayings of a country say a lot about the character of its inhabitants, so he decided to collect some sayings to get a better sense of Hell.
When our speaker returned from his trip, he found a fearsome devil, surrounded by black clouds and looking down on the world of humanity. He wrote down a question "with corroding fires" (3.2).
The gist of the question is this: how do know you that every single bird we see flying through the air isn't a whole world of delight that we're just not able to sense?
Ponder that while we tell you that the reference to "corroding fires" is a sly reference to the Blake-meister himself. He used a copper engraving technique to produce the original plates of this book, which employed his own version of corroding fire to sketch the illustrations—pretty sneaky, there, Mr. B.