What is mutually exclusive collectively exhaustive?

Mutually exclusive collectively exhaustive is a concept in personal epistemology: The best category systems have no overlaps and no gaps.

Example: You're triaging customer support tickets into Billing, Technical, and Account Management. A customer writes: 'I can't log in and I think I was charged twice.' That ticket belongs in two buckets simultaneously — which means your categories aren't mutually exclusive. And if a customer asks about your public API docs, none of those three buckets fit — which means your categories aren't collectively exhaustive. The fix isn't to force every ticket into one category. The fix is to redesign the categories until every ticket lands in exactly one.

This concept is part of Phase 12 (Classification and Typing) in the How to Think curriculum, which builds the epistemic infrastructure for classification and typing.