What is classification system?

Classification system is a concept in personal epistemology: Every category you create determines what you group together and what you separate.

Example: Your email client classifies messages into Primary, Social, Promotions, Updates, and Spam. That classification system determines what you see first, what you check occasionally, and what you never see at all. Switch to a client that classifies by sender relationship — close contacts, acquaintances, unknown — and your entire inbox experience changes. Same messages, different categories, different behavior. The classification system you use doesn't describe your email. It shapes how you respond to it.

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.