Appearance. When it comes to Ligeia, our narrator's obsessed with it. He can remember every line and every curve of his beloved wife's body. And yet, even then, he can't really describe her. There's a certain something that simply can't be captured in words, that lies outside – or maybe inside – of her looks, that's hidden in her expression. It's a strange situation: there's so much description, and yet there's so little to it. Like the narrator's bizarre bridal chamber, appearance may seem to be a simple thing to describe, but if you look at it from a different perspective, it's all too easy to get lost in the details.
In "Ligeia," Poe doesn't simply demonstrate that appearances can be deceiving; for when it comes to Ligeia herself, a physical description can't even begin to capture her essence.
The narrator spends so much time describing Ligeia because her appearance, even in memory, is the only constant thing he knows. Only through those memories can he begin to think about more abstract things.