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.

What is an Identifying Characteristic?

A definition describes a concept.  A concept has many characteristics.  The process of abstraction occurs when we consider the characteristics of a concept individually (or, at least this is one way in which the term "abstraction" is used).  Very often, the characters considered the most when doing definitions are those which describe the essence of the concept.  Consider, for instance, the traditional Aristotelian formula for a definition:

Definition = Genus + Specific Difference

In the historical literature about definitions, emphasis was put on the specific difference as being the essential characters of a concept.  This may serve the philosophers' purposes, but it ignores the problem of identification, and the need to consider identifying characteristics in a definition.

Suppose I want to define the concept of Exit Row in a plane.  Let's try to do so using Aristotle's method.

"A row of seats that provides emergency access to the outside of a plane"

A bit provisional perhaps, but is does begin to convey the essence of an Exit Row.  But how do flight attendants identify an Exit Row to passengers when they give the safety briefing?  It goes something like this:

"For those of you  seated in a row marked by the sign 'No Children In This Row'..."

The essence of an Exit Row is not that it is an area of a plane that is kept free of children.  The essence is that it is a point of emergency egress from a plane.  Only competent adults are thought to be capable of opening an emergency exit, so only they are permitted to sit in an Exit Row.

However, it fairly easy for a passenger  to identify an Exit Row based on finding a sign saying 'No Children in This Row'.  The sign functions as an identifying characteristic.

An identifying characteristic is a characteristic that easily permits the identification of an instance of a concept with the correct concept.

Identification is  a distinct use case (possibly a set of distinct use cases) that definitional work must support.  It is not automatically supported by finding essential characteristics, as our example of Exit Row shows.  We shall return to the topic of identification in the future.  Frankly, identification seems to be poorly supported in the literature of definitions.  Yet it is something we are challenged with every day. 

If practical work with a concept involves identification of instances, the definition of the concept must support this work by clearly listing identifying characteristics and saying how they should be used in identification.