The debate over the status of Pluto indicates that whereas the extension of a word is determined by its intension, the reverse is not also true. Put more simply, the list of objects covered by a word is determined by the list of characteristics that word is thought to describe; on the other hand, the list of characteristics described by a word is not determined by the list of things covered by that word. The objects covered by the word “planet” are determined by what characteristics the word “planet” is supposed to describe, but not the other way around.
At least, that’s what some philosophers argue. Others disagree and argue the contrary: that a word is used to first describe a list of objects thought to be similar in some important ways and then, once this denotation of the word is established, the connotation is developed by teasing out a set of reasonable characteristics from the list of objects. Thus, the connotation is determined by the denotation.
Who is right? Perhaps they both are. An example of how difficult it is to determine this might be the word “tree.” Did people first create a list of tree-like qualities and then later decide which objects go in the list of “trees,” or did people first start calling certain objects “trees” and only later decide what “tree-like” qualities justified inclusion in the list of trees? In logic, science, and philosophy — basically, in any field where very careful thought is required — intension should determine extension. In casual usage, however, it may well be that as a practical matter extension can determine intension.
Meanings Change
The meaning of words can change over time because people will simply use them in different ways, but any change in meaning might represent an extensional change (in what the word denotes), an intensional change (in what the word connotes), or both. For example, the word “marriage” doesn’t currently denote (for most people) any unions between two members of the same sex. If we started to denote such unions by “marriage,” would that require a change in connotation (what characteristics the word intends) or not?
This is, in fact, a key element in the debate over gay marriage. When people disagree over whether gays should be allowed to marry, they are disagreeing in part over the proper intension of the term “marriage.” Unless they come to some sort of agreement over the term’s intension, they will never see eye-to-eye over its extension.
Naturally, if someone is asked for a definition of a word, they can provide vastly different answers based upon whether an extensional or intentional definition is offered. An extensional definition is basically a list of the entities covered by the term — for example listing the planets when asked what a planet is or listing “poem, play, novel, or short story” as a definition of a “fictional work.” Such a definition has advantages because it necessarily contains hard examples of what is being discussed.
An intensional definition, however, lists the attributes or characteristics of the concept — for example, listing the qualities that an object must have in order to qualify as a planet instead of an asteroid. For obvious reasons, this is often easier than an extensional definition because there is no need to list a long series of examples — a list of attributes is always shorter and quicker.
