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[protege-owl] OWL individuals

Thomas Russ tar at ISI.EDU
Mon Apr 23 11:47:51 PDT 2007

On Apr 21, 2007, at 10:02 AM, Alan March wrote:

> Hi.
> Sometime ago I posted a message inquiring as to the intended use of  
> multiple
> Necessary and Sufficient blocks. I did receive some answers, but  
> they were
> mostly examples, and did not quite answer my question.
> So again, when should multiple N&S blocks be used? As far as I have
> reasoned, they should be used when different groups of axioms, each  
> of them
> by themselves or together, allow for a complete definition of a class.
> Horridge et al's Owl Tutorial carries an example of *how* to establish
> multiple N&S blocks, but, at least to the best of my undestanding, not
> *when* to use them. As far as I can gather, it would seem that they  
> must be
> used in the manner I explained above. Thus, an individual may be  
> considered
> a member of the "triangle" class when it *either* "has three angles  
> and is a
> subclass of shape" **or** "has three sides and is a subclass of  
> shape", or
> *both*. When I emphasize "both", I mean to say that if such individual
> fullfilled both N&S blocks, it would also be a member of the  
> triangle class.
> So, my conclusion is that multiple N&S blocks would seem to boil  
> down to a
> sort of "and/or" situation, where a class may be defined as such if it
> carries either block or both blocks. Am I right in this assumption?


Conceptually, I like to think of the necessary and sufficient  
conditions separately.
There are some examples one can come up with where sufficient, but  
not necessary conditions apply.

By separating the necessary and sufficient, one can then make a bit  
more sense of things like the triangle case.

A triangle has 3 sides and 3 angles as necessary conditions.
In other words, every triangle must have both 3 sides and 3 angles.

Having 3 sides is a sufficient condition for being a triangle.

Having 3 angles is a (separate) sufficient condition for being a  

The reason I like to separate such concerns has to do with the way  
inference works.  If all that you know about a polygon is that it has  
3 sides, then that is SUFFICIENT information to conclude it is a  
triangle.  At that point, the necessary condition that it also have 3  
angles comes into play.

As an example of sufficient but not necessary conditions, consider  
the following conditions for being a Student:
    enrolled-in some University
    enrolled-in some Technical-Institute
    enrolled-in some Community-College

The modeling of sufficient, but not necessary conditions in OWL (and  
Protégé) can be done by subclassing and putting both necessary and  
sufficient conditions on the subclass.  Proper subclasses are  
sufficient but not necessary conditions for membership in their  
parent class.

That generally means that most such issues in description logics are  
solved using the class/subclass system.  In the student example, a  
common modeling method would be to have an umbrella concept like  
Institution-of-Higher-Education that subsumes all of the types  
listed, and then have a single N&S condition for that instead.

But one can imagine situations where the individual conditions do not  
warrant having their own class definition, in which case, separate  
sufficient conditions may be justified.

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