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bushy. Some have leaves on their branches while others have needles. And yet, with all this variation,
a person can still recognize an object as a tree. In attempting to answer to this problem, Plato took a
cue from geometry. There is also a great deal of variation between mathematical objects like triangles,
for example. They differ in size, width of the lines making up the sides and angles. But there is also a
template definition that covers all triangles regardless of their differences. They are three sided geo-
metric objects with three angles the sum of which is 180 degrees. So, for trees there must be something
similar. Plato believed that there was some essential concept of
treeness
against which one compares
individual objects and decides whether or not the individual in question qualifies as a tree. But where
did this concept of
treeness
come from ? Here Plato offered a unique, if somewhat bizarre, answer. He
postulated the existence of a nonmaterial world which he call the World of Forms. The World of Forms
contains the eternal unchanging essence of things from the material realm. So that the Form
tree
that
describes the eternal unchanging essence of
treeness
exists in this world and this world can only be
perceived by the intellect. Plato used the term
Form
in much the same way that an information modeler
would use the term entity class. It is the template or general definition of the object type independent
of variations between individual entities.
Plato is seen as a realist. He believed that the World of Forms actually existed independent of minds
to perceive the Forms. Further, he believed that the World of Forms was more real than the physical
world as perceived by the senses because the physical world was unstable and filled with variations
and imperfections. The World of Forms, on the other hand, was a world of timeless perfection. While
the existence of this World of Forms that is somehow more real than the world perceived by the senses
causes one to raise a skeptical eyebrow, it does answer some difficult questions that are not effectively
addressed by later views. For example, how can you have a Form without instances ? And how can
you derive a perfect Form from widely varying imperfect instances.
Plato's goal, in his theory of universals, was to define concepts with the same precision and perfection
that mathematical objects such as a triangle or a square are defined. This is why he had to postulate
the existence of an unchanging World of Forms in which mathematically precise definitions did exist.
Aristotle, Plato's most famous student, rejected the World of Forms based on the obvious problems of
postulating a world that could not be perceived by the senses. Where Plato was interested in ideals,
Aristotle was much more interested in the real world of sense perception. Aristotle's goal, in his theory
of universals, was to classify natural objects into the natural kinds to which they belong and to organize
those kinds into natural hierarchies.
Aristotle saw things a little differently. He did not believe in the independent existence of pure Forms.
The only real entities to him were the particular objects of the world. And yet, Aristotle did not believe
that the construction of universals was purely arbitrary. Trees have properties that are fundamentally
different than the properties of chairs or rocks and these properties are inherent in the individual objects.
They are a part of the physical world and can be known through the senses. Not only do these properties
exist, but they can be articulated and used to organize particulars into kinds. Those kinds can, in turn,
be organized into hierarchies of kinds which Aristotle called genus and species. Hence, universals,
according to Aristotle, exist as collections of properties, and these properties, in turn, do exist in the
real world. Plato and Aristotle were realists, believing that universals have an existence in the world
independent of any minds to perceive them. And this view held throughout the middle ages. However,
the British empiricists, beginning with John Locke, began to recognize the role of the mind in creating
universals and turned the corner from realism to a new view of universals called conceptualism.
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