Smelled. And claimed. In the Grimoire, the Ghosts have been inside the Traveler. Lived inside it basically. They know exactly what it's like. They've been there. They know all about. Knowing > claiming.
English
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Well, of course knowing is greater than claiming. But our ghosts rarely tell us what they know apart from stating the obvious (the whole wizard/moon thing). Are we to dispute the word of someone such as Fenchurch Everis? Are we to care that his main sources of income are teaching bored guardians to dance, or wholesale sparrow-theft?