- dependency inversion: depend on abstractions, don’t depend on concretions.
- Liskov substitution: don’t violate superclass by using is_kind_of; they should return the same type
- does it have one responsibility?
- it is DRY?
- does everything in it changes at the same rate?
- is it open for extension but not for modification?
- does it depend on things that change less often than it does?