This is why I like having stacks of index cards at hand. They’re beautiful and lovely, but if you screw up or make a mistake, it’s just one card. Copy it and throw the imperfect one out if you need to. (Though I find in practice I don’t ever do this.) Because they’re not bound together, you’re also not bound by what you write on one card needing to fit in with what you write on any of the others. There’s no worrying about what subject you’re going to write on this one card tying you to something the way writing in a single subject notebook might. Did this sort of fear exist in the users of 17th century commonplace notebooks, or was it something that evolved in the 20th century with the idea of single subject school notebooks?
My nicest index cards don’t carry the same baggage as my nicest notebooks.