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

26 THE NAMK, THE TERM, THE CONCEPT,
that there are a multitude of stars which agree with
Sirius in giving a twinkling light and in remaining fixed
in relative position to each other, whereas several other
bodies may be seen which resemble Jupiter in giving a
steady light, and also in changing their position from
night to night among the fixed stars. I have now formed
in my mind the general idea of fixed stars by bringing
together mentally a number of objects which agree ;
while from several other objects I have formed the
general idea of planets" This example, from Jevons,
illustrates in a simple case the formation of a universal
by comparison.
We may illustrate the process also by reference to a few
of the general qualities of bodies. Among the qualities
which our sense of sight reveals to us, there is a group con
nected by an obvious resemblance to which we give the name
of
mon to all the different colours, unless we are acquainted
"
colour." It is not easy to explain precisely what is com
with the psychology and physiology of "visual sensation,"
and the physical theory of light ; nevertheless we are con
vinced that they have something in common, and we refer to
this by the general idea named "colour." Similar observa
tions apply to the general idea of "brilliancy." Again, the
universal property of Gravitation, which is common to all the
different degrees of heaviness, is named "
"
weight ;
and
similarly with "density." Now to take a more complex
case. Metals, such as gold, silver, copper, lead, &c., re
semble one another in certain definite ways ; each of them
has colour of one kind or another, each has some degree
more in one case, less in another of brilliancy, weight, and
density : hence the "
universal, metal," includes the general
ideas, "some kind of colour, some degree of brilliancy, of
weight, and of density."
If we pursue the subject scientif
ically, we have of course to include the ideas of other quali
ties in the universal^., that metal is an "element," is a
"good conductor of heat and electricity." Once more, we
observe that some animals walk, others fly, and so on ; that