8. The diptera, or true flies, are readily distinguishable from other
insects by their having a single pair of wings instead of two pairs, the
hind wings being transformed into small knob-headed pedicles called
balancers or halters.
+160. Difficulty of Framing Exact Definitions.+--In order to frame a
logical definition, exactness of thought is essential. Even when the
thought is exact, it will be found difficult and often impossible to frame
a satisfactory definition. Usually there is little difficulty in selecting
the genus, still care should be taken to select one that includes the term
to be defined. We might begin the definition of iron by saying, "Iron is a
metal," since all iron is metal, but it would be incorrect to begin the
definition of rodent by saying, "A rodent is a beaver," because the term
beaver does not include all rodents. We must also take care to choose for
the genus some term familiar to the reader, because the object of the
definition is to make the meaning clear to him.
The chief difficulty of framing logical definitions arises in the
selection of differentia. In many cases it is not easy to decide just what
characteristics distinguish one member of a class from all other members
of that class. We all know that iron is a metal, but most of us would
find it difficult to add to the definition just those things which
distinguish iron from other metals. We may say, "A flute is a musical
instrument"; so much of the definition is easily given.
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