Chuck, (01)
I have more sympathy for modal logic than Pat has, but I am
skeptical about the need for modal logic in the ontology. (02)
CDT> I see that any system that has a non-trivial internal model
> of reality is likely to have modal appreciations of that reality
> as system state changes occur. (03)
I don't know what "modal appreciation" means, but I would agree
that multimodal language is important for many purposes. Examples
include an engineering project, in which there are requirements
('must' or 'shall'), options ('may' or 'can'), obligations
('should' or 'ought'), and all sorts of future or hypothetical
questions. (04)
However, Kripke's version of possible-world semantics is not
convenient for multimodal reasoning, because it very quickly
leads to a tangled mass of "accessibility relations" for every
mode. Furthermore, nobody can ever agree on a good (or even
usable) collection of axioms for all the modal combinations. (05)
My preferred semantics for modal logic is based on a version
by Michael Dunn, in which he focuses on the "laws" that generate
the modalities: For each mode, there is some law or laws that
determine what is necessary or possible (obligatory or permitted). (06)
In ontologies, some people talk about "essential" properties,
but the word "essential" is a disguised modal operator.
With Dunn's semantics, you can get rid of the word 'essential'
by stating a law (or axiom) that makes that property essential. (07)
For a summary of Dunn's semantics and my extensions to it, see (08)
http://www.jfsowa.com/pubs/laws.htm
Laws, Facts, and Contexts (09)
http://www.jfsowa.com/pubs/worlds.pdf
Worlds, Models, and Descriptions (010)
Note that I use the word 'context', but I give it a very simple
formal definition: it's a box into which I put graphs. Then
I show how those graphs can be used to characterize many other
common uses of the word. I won't claim *all* uses of the word,
because there are many uses that are too "creative" to be
characterized in any clear and simple way. (011)
John (012)
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