What is edge case testing?

Edge case testing is a concept in personal epistemology: Items that do not fit neatly into any category expose weaknesses in your system.

Example: A tomato is botanically a fruit but culinarily a vegetable. A virus is neither alive nor dead by conventional definitions. A freelancer is neither employee nor business owner in most tax codes. When you encounter something that doesn't fit your categories, you haven't found a defective item — you've found a defective boundary. The item is data. The discomfort is signal.

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.