What does it mean that classification is how you carve reality into categories?

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.

Try this: Pick one domain of your life you actively manage — your task list, your bookshelf, your notes, your contacts. Write down the categories you currently use. Then invent a completely different classification system for the same items — organize by urgency instead of project, by emotional weight instead of topic, by how recently you engaged instead of alphabetical order. Sort ten items into both systems. Notice what each system makes visible and what it buries.