There are a lot of topics compressed into this comment, so I am only going to pick one here. It is the different classes of identification that should be captured in a definition. I suggest that these are:
- Characteristics of the concept being defined that set it apart from other related concepts. These are the classic specific differences (differentia)
- Characteristics that can be used to recognize an instance of the concept. This is what I was trying to highlight in the original post when I stated that an exit row in an airplane could be recognized by a sign saying "no children in this row". There is no reason for these characteristics to be specific differences.
- Characteristics that can be used to identify an instance of the concept. This is what Stijn was talking about, saying his passport could be used to identify him. Identifying an instance is not the same as identifying or recognizing a concept.
The per-application rules that Stijn mentions add more complexity, but that will require another post.