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Socrates, Plato, and other individuals.

The single objects are called INDIVIDUALS, because the division cannot
be carried farther. The highest class is technically the SUMMUM GENUS,
or _Genus generalissimum_; the next highest class to any species is
the PROXIMUM GENUS; the lowest group before you descend to individuals
is the INFIMA SPECIES, or _Species specialissima_.

The attribute or attributes whereby a species is distinguished
from other species of the same genus, is called its DIFFERENTIA or
DIFFERENTIAE. The various species of houses are differentiated by their
several uses, dwelling-house, town-house, ware-house, public-house.
Poetry is a species of Fine Art, its differentia being the use of
metrical language as its instrument.

A lower class, indicated by the name of its higher class qualified by
adjectives or adjective phrases expressing its differential property
or properties, is said to be described PER GENUS ET DIFFERENTIAM.
Examples: "Black-bird," "note-book," "clever man," "man of Kent,"
"eminent British painter of marine subjects". By giving a combination
of attributes common to him with nobody else, we may narrow down the
application of a name to an individual: "The Commander-in-Chief of the
British forces at the battle of Waterloo".

Other attributes of classes as divided and defined, have received
technical names.

An attribute common to all the individuals of a class, found in that
class only, and following from the essential or defining attributes,
though not included among them, is called a PROPRIUM.

An attribute that belongs to some, but not to all, or that belongs to
all, but is not a necessary consequence of the essential attributes,
is called an ACCIDENT.

The clearest examples of Propria are found in mathematical figures.
Thus, the defining property of an equilateral triangle is the equality
of the sides: the equality of the angles is a proprium. That the
three angles of a triangle are together equal to two right angles is
a proprium, true of all triangles, and deducible from the essential
properties of a triangle.

Outside Mathematics, it is not easy to find _propria_ that satisfy
the three conditions of the definition. It is a useful exercise of
the wits to try for such. Educability--an example of the proprium
in mediaeval text-books--is common to men, and results from man's
essential constitution; but it is not peculiar; other animal