# 1999/5/5

#===========================#
# definitions & hierarchies #
#===========================#

One of the reasons a concept-hierarchy is useful is that
it explicitly gives an ostensive definition of every concept.
For example, the (partial) hierarchy

existent
    entity
        animal
            person
                man
                    Dick
                    Bob
                woman
                    Sue
                    Pauline
        plant
        object

includes the ostensive definitions

entity is either animal or plant or object
person is either man or woman
man is either Dick or Bob
woman is either Sue or Pauline

However, some of these definitions are of no use
outside the context of the current finite sample,
i.e., the units

Dick, Bob, Sue, Pauline

As we add new units, we need a genus-differentia definition.
For example,

man is person with sex=male
woman is person with sex=female
Reno has sex=male

shows that

Reno isa man

Wrong.  Those of you who remember my previous examples
will "smell a dog" at this point.  Yes,

Reno isa dog
dog isa animal

So we need genus-differentia definitions for all our concepts,
except existent.  In the context of imaginary worlds, it's
helpful to keep in mind a "definition" of existent, such as

existent is anything that exists in reality

Just a reminder:
definitions are relations, and are part of the identity of a concept.

One final observation:
An ostensive definition is a "definition by integration".
A genus-differentia definition is a "definition by differentiation".
We can also have a "definition by identity", e.g.,

female person is woman
morning star is evening star