Why does Betteredge say that drunkenness nor sleepwalking can explain it?

Betteredge says this because no explanation provides how the paint got onto the nightgown. So basically she is saying that there is no explanation.

Betteredge points out that even if Franklin had been drunk, or sleepwalking, when he took the diamond, that doesn't account for what happened since then. How did the diamond get to London to be pledged with Mr. Luker?

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