The neatest classification often leaves the most interesting case outside.

A tidy classification is tidy because the awkward cases were left out. The thing that fits two categories, or neither, is precisely where the interesting information sits, and it is exactly what a clean scheme has to discard in order to stay clean.

When a framework feels satisfyingly neat, go looking for what it cannot hold. Ask what falls between the categories and whether that group is small or actually quite large. The exceptions are usually where the next real understanding is, so keep a list of them rather than rounding them away.