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. . . . . the insertion of the words `and equal' leaves, and indeed leads, a reader to
suppose that there may be a four-sided figure whose opposite sides are parallel and
not equal. Much is often inferred in this manner which was by no means in the
Author's mind; thus, he who says that it is a crime for people to violate the property
of a humane landlord who lives among them, may perhaps not imply that it is no
crime to violate the property of an absentee landlord, or of one who is not humane;
but he leaves an opening for being so understood.
Many attempts at definition fail because they do not name the genus. It is no
definition to say, "So and so is a a thing which", etc., or "So and is when", etc. "A thing"
and "when" do not state the genus and lead to nowhere. At school most of us learned that
"a noun is the name of anything", but this definition fails to give the genus, for it does not
make clear that a noun is a word. How should we define the word "rock"? "A rock is
anything which forms part of the earth's crust" fails because it does not name the genus.
But "A rock is a material substance which forms part of the earth's crust" would be a
correct definition.
The first rule of definition is:--
"A definition should not contain more than the connotation of the term in question" (Venn).
Suppose I were to define a student as "A human being residing in an educational
institution, and devoted to the pursuit of knowledge", I should violate this rule. The
undue limitation of the class by the quality "residing in an educational institute" makes
the definition worthless.
A further rule is:--
"A definition should not contain less than the full connotation" (Venn).
The following is an example of failure to observe this rule: "A map is a representation
of some part of the universe." This definition fails in two respects: it lacks the word
pictorial and unnecessarily includes too much, namely, "the universe":--
"This is perhaps the commonest fault of any, as we are apt to feel satisfied if our
definition covers the case we have in our immediate view, and to omit to examine
whether it does not also admit something else of which we were not at the moment
thinking" (Venn).
"A definition must not contain the name defined", says Jevons, for by subjecting a
term to definition we assume that it is not thoroughly understood, and the repetition of the
unknown term cannot, therefore, make it known.
The following are examples of definitions faulty in this respect:--
A noun is the name of anything.
Life is the sum of vital functions.