[56]
Genus is of little use when we desire to prove a
species, but of great value for its elimination. A tree
is not necessarily a plane tree, but that which is not
a tree is certainly not a plane tree; again, a virtue is
not necessarily the virtue of justice, but that which
is not a virtue is certainly not justice. We must
proceed from the genus to the ultimate species;1 for
example, to say that man is an animal will not
suffice; for animal merely gives us the genus: nor
yet will the addition of the words “subject to
death” be adequate; for although this epithet gives
us a species, it is common to other animals as well.
If, however, we define man as a rational animal, we
need nothing further to make our meaning clear.
1 cp. § 5.
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