318. We will first define these terms in the sense in which they are
now used, and afterwards examine the principle on which the
classification is founded and the sense in which they were originally
intended.

(1) A Genus is a larger class containing under it smaller
classes. Animal is a genus in relation to man and brute.

(2) A Species is a smaller class contained under a larger one. Man
is a species in relation to animal.

(3) Difference is the attribute, or attributes, which distinguish
one species from others contained under the same genus. Rationality
is the attribute which distinguishes the species, man, from the
species, brute.

N.B. The genus and the difference together make up the Definition of
a class-name, or common term.

(4) A Property is an attribute which is not contained in the
definition of a term, but which flows from it.

A Generic Property is one which flows from the genus.

A Specific Property is one which flows from the difference.

It is a generic property of man that he is mortal, which is a
consequence of his animality. It is a specific property of man that
he is progressive, which is a consequence of his rationality.