How can knowing something be sinful? - Philip Pullman

Lyra at eighteen sitting intent and absorbed in Duke Humfrey’s Library with the alethiometer and a pile of leather-bound books. Tucking the hair back behind her ears, pencil in mouth, finger moving down a list of symbols, Pantalaimon holding the stiff old pages open for her … “Look, Pan, there’s a pattern there — see? That’s why they’re in that sequence!” And it felt as if the sun had come out. It was the second thing she said to Will next day in the Botanic Garden.

I have stolen ideas from every book I have ever read.

Must is not the same as can, Lyra'.
'But if you must and you can, then there's no excuse!