Store literature notes (notes you take while reading) separately from the main notes.
Literature notes are not your ideas, and you might not even agree with them.
Treat them in the same way you treat fleeting notes—process them and insert separate ideas into the main slip box.
Additionally, store literature notes with references in your bibliography slip-box.
This helps keeping slip-box organized for developing ideas: Zettelkasten is about developing ideas, not collecting them