An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

DEFINITION. 173
be familiar to other minds. The nearest genus {genus proximuiri)
is referred to because then a simpler specific difference
is sufficient to distinguish the object from that genus.
So far, we have treated definition only as a means of
marking off an object from others ; and this is the etymo
logical meaning of the term (definitio, opos, opicr^os}. The
definitions with which we begin an inquiry, in any branch
of knowledge, must be of this kind ; and whatever breadth
and depth may be given
not the beginning of the inquiry
to the definition afterwards is
but the result of it. Ob
viously it is of great importance, at the beginning of some
theoretical or practical discussion about a certain matter,
that we should be able to mark it off by characteristics which
are precise and easy to find. At first this is of more con
sequence than any reference to characteristics which are
scientifically more profound. In scientific treatises we
often find objects referred to by properties which are com
paratively unimportant but not easily mistaken.
5. The following rules for the expression
and for
mulation of definitions are based on those given by
Aristotle.
(i) The fundamental rule is that the definition must
state the most essential features of the objects to which
the term is applicable.
Aristotle considered that the definition per genus et
differentiam secured the statement of the essential
features. But from the modern point of view this is
not so. We have seen that such definition may be
nothing more than a preliminary survey of the ground.
From the modern point of view, also, Aristotle made
too complete a separation between the "essential" and
the
"
"
accidental qualities of objects. For the present
we may say that the essential qualities are those without
which the thing could not be what it is. A man, for
instance, by living alone for years on a deserted island,
might lose the essential qualities of manhood and