thinker. This, however, is just what Plato meant.
They are not subjective ideas, that is, the ideas in a particular and
existent {189} mind. They are objective Ideas, thoughts which have
reality on their own account, independently of any mind.

Fourthly, each Idea is a unity. It is the one amid the many. The Idea
of man is one, although individual men are many. There cannot be more
than one Idea for each class of objects. If there were several Ideas
of justice, we should have to seek for the common element among them,
and this common element would itself constitute the one Idea of
justice.

Fifthly, the Ideas are immutable and imperishable. A concept is the
same as a definition. And the whole point in a definition is that it
should always be the same. The object of a definition is to compare
individual things with it, and to see whether they agree with it or
not. But if the definition of a triangle differed from day to day, it
would be useless, since we could never say whether any particular
figure were a triangle or not, just as the standard yard in the Tower
of London would be useless if it changed in length, and were twice as
long to-day as it was yesterday. A definition is thus something
absolutely permanent, and a definition is only the expression in words
of the nature of an Idea. Consequently the Ideas cannot change. The
many beautiful objects arise and pass away, but the one Beauty neither
begins nor ends. It is eternal, unchangeable, and imperishable. The
many beautiful things are but the fleeting expressions of the one
eternal beauty. The definition of man would remain the same, even if
all men were destroyed. The Idea of man is eternal, and remains
untouched by the birth, old age, decay, and death, of individual men.

Sixthly, the Ideas are the Essences of all things. The definition
gives us what is essential to a thing. If we {190} define man as a
rational animal, this means that reason is of the essence of man. The
fact that this man has a turned-up nose, and that man red hair, are
accidental facts, not essential to their humanity. We do not include
them in the definition of man.

Seventhly, each Idea is, in its own kind, an absolute perfection, and
its perfection is the same as its reality. The perfect man is the one
universal type-man, that is, the Idea of man, and all individual men
deviate more or less from this perfect type. In so far as they fall
short of it, they are imperfect and unreal.

Eig