Kitap 5
10.56
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; cp. § 5. 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.
10.57
On the other hand species will give us clear proof of genus, but is of little service for its elimination. For example, justice is always a virtue, but that which is not justice may still be a virtue, such as fortitude, constancy or self-control. Genus therefore cannot be eliminated by species unless all the species included in the genus be eliminated, as for instance in the following sentence: That which is neither rational nor mortal nor an animal is not a man.
