> -----Original Message-----

> From:

[hidden email]
> [mailto:

[hidden email]] On Behalf Of

> Thomas Russ

> Sent: Monday, April 23, 2007 3:48 PM

> To: User support for the Protege-OWL editor

> Subject: Re: [protege-owl] OWL individuals

>

>

> 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?

>

> Yes.

>

> 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 triangle.

>

> 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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