Database Reference
In-Depth Information
Defined classes are classes whose restrictions on properties are both necessary
and sufficient conditions. What this means is that anything having that combination
of properties must be the defined class. This is best expressed by the Rabbit state-
ment “Anything that.” Consider the following:
A Wood is anything that:
is a kind of Landform;
has dense cover of Trees.
Class: Wood
EquivalentTo: Landform and
hasDenseCover some Trees
What this is saying is that any piece of land that has dense tree cover must be a wood.
In terms of necessary and sufficient conditions then, we can say that to be a wood
it is necessary for it to be a piece of land and for the land to be densely covered by
trees, and that being land that is densely covered by trees is sufficient to be a wood.
Rather than merely describing the class, we have defined it. Another way to look at
this is that the class Wood is equivalent to the class of things that are land with dense
tree cover.
As we can see, the difference in OWL between a Defined Class and one that is
merely described is that the equivalence rather than subclass relationship is used.
Defined classes are quite powerful because a reasoner will classify any individual
that meets the necessary and sufficient conditions as being a member of the defined
class, irrespective of whether we have explicitly stated its membership. For example,
any other thing that is defined as land and has dense tree cover would be automati-
cally classified by a reasoner as a wood. This can be very useful, but equally it is
very easy to get it wrong; we have to be really sure that what we say is true. As an
example, consider a Bakery. We might want to define a Bakery as follows:
A Bakery is anything that:
Is a kind of Place;
Is intended for Baking of Bread.
Class: Bakery
EquivalentTo: Place that
isIntendedFor BakingOfBread
So, any place that has a purpose of baking bread is a bakery, a statement that
sounds quite reasonable. However, consider a large supermarket that also bakes its
own bread:
Every Supermarket is a kind of Place.
Every Supermarket is intended for the
Sale of Groceries.
Medina Merea Superstores is a
Supermarket.
Medina Merea SuperStores is intended
for Baking of Bread.
Class: Supermarket
SubClassOf: Place that
isIntendedFor SaleOfGroceries
Individual:
MedinaMereaSuperstores
Types: Supermarket
Facts: isIntendedFor some
BakingOfBread
What this says is that the particular shop Medina Merea SuperStores (the Medina
Branch of the Merea SuperStores chain) is a supermarket, and it also bakes bread.
From the fact that it is a supermarket, we know it is also a place. This means that
it has both the necessary and sufficient conditions also to be classified as a bakery
(the fact that it has additional conditions is irrelevant). For some people this might
