Re: Rigid Properties

[Nicola Guarino <Nicola.Guarino@ladseb.pd.cnr.it>]

At 10:05 AM +0200 22/6/2000, Matthew West wrote:

>We have had a similar discussion in EPISTLE (we called them "essential
>classes").

Indeed the adjective "essential" may be more appropriate than "rigid" 
to characterize what we have in mind (Chris Menzel made the same 
terminological observation some time ago). We continue to use "rigid" 
just for simplicity. However, notice that an "essential property" may 
be essential for one instance but not for another, that's why a more 
complicated term must be invented.

By the way, better talking of "properties" rather than "classes". A 
property can correspond to multiple classes (its extensions) in 
different worlds/situations. This is related to the modality 
discussion, however (different thread).

>
>We also had difficulty with the sort of example Nicola gives such as a bomb.
>This is to do with the role something plays. We found that physical objects
>that are artefacts generally have a purpose, or intended role. This is often
>in the name of the thing. So for a bomb, its intended purpose is to blow
>things up. Even if I use it as a door stop (its actual role) its intended
>role is still to be a bomb (so here we part company with what Nicola was
>saying). So being intended to blow things up (intended role) is an essential
>class (rigid property) for a bomb.

Indeed all examples with artifacts are tricky. I agree that in many 
cases is useful to take the intended role as an essential property of 
artifacts.  So we may postulate a difference between a stone 
intentionally shaped to be used as a hammer and a stone that 
accidentally has that shape. Saying that "hammer" is rigid means that 
no accidental hammer exists.

In the case of the bomb, I agree that it is essentially intended to 
blow things up, but in my example I imagined that something that 
blows things up may not be essentially a weapon: so "being a bomb" 
may be a rigid property, and "being a weapon" a non-rigid one... 
Another example is a knife: it may be a weapon, it may not (while 
remaining essentially a knife).

These reflect only particular points of view, of course. The 
important thing is that these points of view can be clarified by 
using the rigid/non-rigid distinction.

-- Nicola

P.S.: Sorry for my slow reactions to the very interesting messages 
appearing on this list...


  ---------------------------------

Nicola Guarino
National Research Council	phone: +39 O49 8295751
LADSEB-CNR		fax:   +39 O49 8295763
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(***updated 2/6/2000***)