Re: FAQ: stratified class hierarchies vs. RDFS

Dan,

  There is a rather fundamental difference in the intended meaning of 
rdfs:Class vs Sets. One comes from cognitive science and the other from 
Math.  rdfs:Class is intended to capture the concept of "category" or 
"kind" as that term is used in cog-sci and not the concept of Set. In 
Cyc, for example, we had different nodes corresponding to Class and Set. 
(Quine wrote a very nice essay on this topic [1]).

 Here are some examples that illustrate the difference. (V is the union 
operator).
a) Both perspectives would include the concepts of Person and Table. The 
rdfs:Class perspective would not  include (Person V  Table). The Set 
perspective would.
b) DanC is an instanceOf Person. In the set perspective, he would also 
be an instanceof (Person V SquareTriangle) and (Person V 
SuperNovasOnEarth) ...

Saying that rdfs:Class is the rdfs:Class of all rdfs:Classes does not 
cause problems because we do not and cannot have a theory of 
rdfs:Classes such as ZF set theory.

 Both these concepts are very useful and we need them both. But it is 
important not to mix up the two. Both approaches are relatively common, 
with the rdfs:Class approach being more commonly used in large scruffy 
implementations and the set oriented approach being more common in 
formalizations such as DLs.

The important question is, which one do we use to describe concepts like 
"Person"? My personal preference is for the cog-sci approach. It is more 
pliable and fairly immune to logical nastinesses like paradoxes. I would 
also argue that this robustness also makes it a better choice for the SW.

Guha

[1] Quine, W. V. O. (1969). Natural kinds. In Ontological relativity and 
other essays, pages 114--138. Columbia University Press, New York, NY.

Received on Friday, 21 June 2002 14:32:02 UTC