It would be great if we could tag sections (sentences, paragraphs) within books so that we could later quickly collate all that we've come across on a particular topic or theme from an author or book or books.
For instance, suppose one is reading Plato's Republic and they come to his remarks on the soul having three parts. It would be great if we could select that "paragraph" and then add a tag to it: "Soul"
P.S. And if anyone has some knowledge of how we might be able to do something similar please let me know.