1. Plato. Ideas enter philosophy in the dialogues of
Plato. They are an answer to the question, “What do
common nouns signify?” though that is not Plato's way
of putting it. A common noun is obviously peculiar,
as it groups a number of things together instead of
differentiating an individual from all other individuals.
One uses the adjectives which correspond to these
nouns in much the same way. One speaks of a picture
as “beautiful,” of an act as “kind,” of hypocrisy as
“evil,” of apples as “red,” and in doing so one classifies
the objects and acts referred to. But the moment this
is done it is admitted that some properties or qualities
are common to a number of things. Several pictures
may be “beautiful” and things other than apples may
be “red.” The common property therefore is not re-
stricted to any place or date and hence is entirely
different from a material object.
We learn as children that spatiotemporal location
is an essential characteristic of material things. We say
that two material objects cannot occupy the same
space at the same time and that a single material object
cannot be in two places at the same time, a bit of
information which is the basis of an alibi in court. But
a large number of common properties may be present
in a single object and a single common property may
characterize a large number of objects at the same
time. We learn the consequences of this when we study
geometry in school and are warned not to argue from
the looks of the figures that we draw. The triangle on
paper, we are taught, is not the triangle we are discuss-
ing; it is an imperfect representation of the geometri-
cal triangle.
It is possible that Plato's theory of ideas was sug-
gested by these geometrical forms. If we are not study-
ing the triangle drawn on paper, what one are we
studying? The answer usually is: “Something defined
by abstract rules,” or “A plane figure bounded by three
straight lines.” But there are no real planes or straight
lines in human perception and hence we resort to
saying that such things are intellectual constructs, ab-
stract concepts, creations of the mind. Yet we also insist
that they are not imaginary beings like mermaids or
unicorns, for once defined they follow certain laws,
imply certain consequences, and actually control our
thoughts rather than obey them. The sum of the angles
of a plane triangle has to be 180 degrees, neither more
nor less, and we cannot by an act of will or of fantasy
change it. This, however, is true of all common prop-
erties. Once we know what we mean by “beautiful,”
“evil,” “red,” “kind,” we are committed to certain
other assertions which those adjectives presumably
entail.
Plato then is first convinced that ideas cannot be
spatiotemporal like material objects. If they are not
material, some other term must be found for them.
“Mental” will not do since, as we have said, that would
put them in a class with volitions, fantasies, feelings,
and so on. The term used was “eternal” or “timeless.”
The idea of a triangle may be discovered at a certain
date and in a certain place, but it did not come into
being at that place and date. This seems to be true
of all common properties and if so, they may be said
to form a realm of their own existing apart from the
material world. Their interrelations are not causal but
logical. The nature of a plane surface does not cause
three straight lines to be the smallest number to enclose
a plane figure, but it implies that the smallest number
of straight lines that may bound a figure is three.
This being so, no ideas can be known by our sense-
organs. Just as we do not see real triangles, we do not
see real kindness, evil, beauty, or redness. We see kind
acts but know that they are kind; we see evil deeds
but know that they are evil, and so on. We ask ourselves
whether a certain deed which we observe is really good
or not and it is reflective thought, not perception, that
gives us the answer. The process of reflection is exhib-
ited in the Platonic dialogues. A question is asked, an
answer given, the answer is analyzed to find out if it
is logically consistent, and if it is not, it is rejected.
Many of the dialogues, called by the ancient editors
“dialogues of search,” end at this point.
The ideas then determine what the particular objects
are. It is, for instance, the presence of evil in an act
that makes it evil, the presence of beauty that makes
a picture beautiful. This is a cardinal point in Plato's
theory of ideas and the relationship of the particular
thing to the ideas that characterize it was called “par-
ticipation.” The picture participates in beauty. But
how a spatiotemporal object can participate in an
eternal being was left unclarified. When the matter
is discussed in The Sophist, participation is named and
the limitations of participation defined, but it is clear
that the gap between what was later—by Philo
Judaeus—called the Intelligible World and the Sensible
World was simply accepted by Plato as a fact.
If we cannot perceive the presence of an idea in
our ordinary experience, how do we know what we
are experiencing? The answer was given in the form
of a myth in the dialogue Meno. Since we cannot
apprehend an idea through our sense-organs, we must
come into the world with a stock of them. These are
the innate ideas. They are probably of a mathematical
nature for the most part—ideas of equality, identity,
difference, and similar relations. In the myth it is said
that the soul before birth was in the world of ideas
and that it retained a dim memory of some of them
after birth. As we grow up, the myth continues, on
certain occasions we are reminded of what we knew
in our prenatal life, and we are able to light upon the
idea that fits the case before us. The mythical details
of this need not detain us, though much was made of
them in the Renaissance by the Florentine Platonists,
and they survived to be incorporated in Wordsworth's
Ode on Intimations of Immortality (1806). What is of
greater importance is the suggestion that the human
mind is limited to certain ways of thinking, to certain
logical rules, certain categories of existence, and that
it awakens to these as its education progresses.
This appears in our own day in that theory of teach-
ing known as the “Socratic Method,” according to
which the teacher serves to bring out of the pupil's
mind that which lies hidden in it. In Aristotle these
categories were to be named and in the nineteenth
century they were “deduced” by Kant and his follow-
ers. But as far as Plato is concerned, they are traits
of all ideas and are utilized whenever a person reasons.
Thus all ideas are differentiated from some others; some
are harmonious with some others; some are implied
by some others and naturally imply others. These traits
are known instinctively. Anyone knows that if some-
thing is good, it cannot also be bad at the same time
and in the same respect. But only dialectical training
will teach a person to apply such information in actual
cases of reasoning.
The ideas, since material things participate in them,
are models by means of which we can judge the ade-
quacy or perfection of things. If we know, for instance,
what justice is, we can discover how closely a given
state approaches justice in its constitution. What such
a state would be like is described twice by Plato, once
in The Republic and once in The Laws, his last work.
What is relevant here is the technique by which the
idea of justice is defined. After disposing of a popular
and false definition, Plato begins afresh and comes up
first with the statement that whatever justice may be,
it has terminal value, i.e., it is good in itself and its
by-products, such as happiness, need not be considered.
Hence if just men suffer at the hands of their fellows,
this is of no more importance to the nature of justice
than are the imperfections in the drawings of circles
made in the sand. An idea is always to be discovered
by the dialectical method of question and answer, and
Plato is convinced that a false definition will reveal
its falsity by its inner inconsistencies. We must try to
approximate an idea's perfection as closely as possible,
though knowing that the task is hopeless in real life.
At this point one sees that the idea has become what
we would call the “ideal,” and that the dualism be-
tween the ideal and the existent has widened. This was
to lead to attempts in post-Platonic philosophy to
reduce the duality. And since the ideas or ideals were
accessible only to the reason, two kinds of cognition
were established, perception which grasped the tem-
poral, changing, multiple things in the material world
without knowing what it was grasping, and reason
which knew the eternal, immutable, unified things of
the ideal world. It was the latter world that was “really
real,” the former being a dim reflection of it. Since
only a few human beings, the philosophers, were capa-
ble of reasoning, they would be in charge of the state
if the state were just and perfect. The majority of men
are either led by their appetites or their passions, but
both should be controlled by the reason. The trouble
with society as it exists is that appetitive and passionate
men, who confuse opinion with knowledge, are at its
helm. The Republic and The Laws discuss what must
be done to put them in their proper places.
The ideas then have at least the following properties:
they are universals, class-characters, analogous to
mathematical figures; they are timeless and unchang-
ing; they are ideals, not existent objects in space-time;
they are known only to the reason.
