An Introduction to Critical Thinking and Creativity - always yours

28 DEFINITIONS
3.5.2 Definition by ostension
An ostensive definition explains the meaning of a term by giving examples, as
when we explain the meaning of red to children by pointing to examples of red
things. Or someone might explain a professional as "people like doctors, lawyers,
and accountants." Ostensive definitions are useful when it is difficult to explain
the meaning of a term precisely. But an ostensive definition is a form of definition
by extension, and intensional definitions can explain meaning better if they are
available.
3.5.3 Definition by genus-differentia
The genus-differentia method is a very useful tool for constructing definitions.
According to this approach, when we want to define a term, first we identify the
broad category that the term is supposed to apply to. This category is known as
genus. To define mule, we note first of all that a mule is an animal. Of course,
there are many kinds of animals other than mules. So the next step is to identify
the differentia. This is the property that separates the items in the genus into two
groups—those that fall within the extension of the definiendum, and those that
do not. In the case of mule, this would be the property of being the offspring of a
male donkey and a female horse. Putting the two parts together, we arrive at the
full definition:
• A mule is an animal that is the offspring of a male donkey and a female
horse.
This type of definition is useful because it informs the audience about the kind
of thing the definiendum applies to, even if the complete definition is difficult to
remember and understand. Here are more examples of this kind of definition,
with the genus part underlined:
• Ice = frozen water.
• Witness = a person who testifies under oath at a trial or deposition.
• Vitamin = a low molecular weight organic compound required in trace amounts
for normal growth and metabolic processes.
• Bear market = a prolonged condition of the financial market in which investment
prices fall and accompanied by widespread pessimism.
• Phoneme = the smallest unit of sound in a language.
3.6 THREE MISCONCEPTIONS ABOUT DEFINITIONS
Definitions are no doubt useful, and this has led some people to claim that we
should define all the words we use. But this is far too extreme. First of all, we do