Three Classes of Identification in a Definition

Stijn commented on my earlier post "What is an Identifying Characteristic?" (http://definitionsinsemantics.blogspot.com/2011/11/what-is-identifying-characteristic.html) raising the point that "identification of a thing is dependent on the application".  He lists out things that identify him, and notes that one cannot always be substituted for another.  E.g. a passport cannot always be substituted for a driving license.  It depends on the application, and each application has rules about what can be used as identification.  Stijn asserts that trying to capture all such rules in a definition will create conflict between the parties representing the applications.  So he advises us to separate a definition from capturing such rules.

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. 

So it turns out that identification is quite complex in definition work.  Simply talking about "identifying characteristics" as I did, does not take this richness into account.

The per-application rules that Stijn mentions add more complexity, but that will require another post.