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

IQ4 THE CATEGORIES OR PREDICAMENTS.
"
"
"
natural kYnds or real
"
kinds were held to be separated
from one another by a practically infinite number of differ
ences in other words, they are at bottom different and
separate. Hence arose the importance attached to the
scheme of predicables given by Porphyry, and to such
arrangements as the Porphyrian tree. The natural kinds
were supposed to have been fixed at the beginning of
l
things ;
" human
"
for beings," instance, constituted a natural
kind" in this sense. Hence when we conformed our con
cepts to the distinct kinds which Nature shows us, any
arrangement of the concepts, such as the Porphyrian tree,
had a scientific significance, it dealt directly with relations
of real things ; and when seeking for summa genera, we
were really investigating the fundamental differences in
Nature.
It will be advantageous
to have a clear answer to the
question How much of this theory is still tenable ?
The rigid notion of natural kinds as mutually exclusive
or, as the Greeks would have said, of eftfr;, species, as
mutually exclusive arose like other peculiarities of Greek
Logic, because Geometry, as then understood,
the type and model of genuine Science. In
was taken as
Greek Geom
etry, in Euclid, for instance, divisions or classes like circle,
Polygon, or like figure, line, were rigidly cut off from one
another ; there was no conceivable passage from polygon
to circle, from ellipse to circle, from figure to line. But
according to modern Geometry, a circle may be conceived
as an ellipse whose foci coincide, or as a polygon with an
infinite number of sides ; similarly, by conceiving of a
triangle
in which the difference between two sides and the
third is infinitesimal, so that one angle =180 and the other
two=o, we reach the straight line. Hence there may be
a geometrical evolution of one figure out of another ; but
the possibility of this does not take away the meaning of
the
"
"
real kinds of figure indicated by the names circle,
polygon, &c.
1 In later times these natural kinds were believed to be due, in the
animal and vegetable kingdoms, to special acts of creation, all the
members of the same "kind" having descended from the same
parents.