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urges the absolute, irrefragable rule of logic, that the specific difference 
(differentia) is restricted to but one specific mark. In supPort of this 
position he uses Sir William Hamilton’s authority against me. It be- 
hooved him to be very sure of this rule, before he mercilessly employed 
it as a pestle to bray mein his mortar. But— 
(1) The enforcement of this rule would convict distinguished logicians 
of ignorance, ruin some of their definitions, and make sad havoc of others 
which are as dear to Dr. Martin as the blood of his heart. A few illus- 
trations must suffice. Bowen says: 
The intension of every species contains the genus—that is. the agere- 
gate of marks which characterise the genus—and the specific difference— 
that is, the aggregate of marks by which this species is distinguished, 
both from the genus to which it is subordinate, and from the other spe- 
cies with which it is co-ordinate . . . species and genera may be per- 
fectly discriminated by one or few characters. 
Jevons says: 
It is evident, therefore, that there must be more qualities implied in 
the meaning of the species than of the genus, as well as a certain addi- 
tional quality or qualities by which the several species are distinguished 
from each other. Now these additional qualities form the difference, 
which may be defined as the quality, or sum of qualities, which mark 
out one part of a genus from the other part, or parts. 
These must serve as samples. Take now a practical specimen of defi- 
nition by an old logician of whom Sir William Hamilton speaks with 
great respect, and whose work was a text book with John Owen and the 
dissenters of his day. Burgersdyck, in his Institutionum Logicarum, 
is defining logic itself. He proceeds by the genus and the difference. 
The genus is art. ‘The difference,” he says, “is taken from the end, the 
office (!) and the matter or object.” Here are three specific marks in the 
specific difference, and among them “office,” for the use of which in the 
differentia of my definition, Dr. Martin chastises me. 
But to come home: Let us take the definition of the Westminster 
Shorter Catechism. Begin with the definition of God. In this cele- 
brated definition you have a number of specific marks con-noting along: 
with the generic, the adorable subject. After pronouncing this “the 
best definition in a brief compass,” Dr. Thornwell says: 
Here the genus to which the substance of God is referred is spirit, in 
strict accordance with the Scriptures and the manifestations of his nature 
which are wade by his works; the difference, those qualities which be- 
long to spirit in its full and normal development, heightened beyond all 
bounds of conception by terms which are borrowed from God as an ob- 
ject of faith. 
