Someone was right about the facts and ruined by them: the theory wasn't the problem; the relationship was.

Arne-Sayles is the philosopher whose system exists only in commentary, whose followers ruin themselves for it. His theory was true, there really are other worlds, and the House is one of them. And it still ate its believers, because being right about the House is not the same as belonging in it. Ketterley had the correct theory and the wrong relationship; Piranesi has no theory and the right one. The final turn: a belief can be true and still destroy you, if you arrive as an exploiter instead of a guest.