Category vs Part

MANUAL MODEL. From CARS the manual shows both descents: classes/categories (BMW, Pontiac, Fiero, GT) and parts (wheels, doors, hub caps, lug nuts). Both are 'down', and they behave completely differently. A Fiero can do a car's job; a lug nut cannot. Mixing them produces hierarchies that look fine and reason badly.

• Downward is not one relationship but at least two
• The relation should be tagged, not assumed
• Some items are genuinely both, depending on context

In goal work this matters: 'a chapter' is a part of a book, 'a novel' is a kind of book. Parts give you sequence; classes give you alternatives.

Say the sentence out loud with each connector and notice which one holds.

• “Is that a kind of it, or a part of it?”
• “Could it do the whole thing's job?”
• “If you removed it, is the whole damaged or just different?”

Practitioner judgement: challenge what is useful, not every pattern you can hear.

Tag every downward move. When the answer is genuinely context-dependent — a wheel is a part of a car and a member of the class 'wheels' — say so rather than forcing one answer.

Client autonomy first — offer language the client can verify and decline.

• Building a ladder that silently switches from classes to parts
• Insisting on one right answer where ontology is ambiguous
• Treating 'smaller' as automatically meaning 'part'