>Pat, Chris, Kathy, and Barry,
>
>The problem of stating necessary and sufficient conditions
>for defining anything is nontrivial, even in mathematics.
>For phenomena in nature or the results of typical human
>behavior, definitive statements are problematical, to say
>the least.
>
>Belief revision systems, database systems, and knowledge-based
>systems distinguish levels of "entrenchment" (whether or not
>they use that term), and I believe that an ontology should also
>make such distinctions at the metalevel. (01)
Suppose we were to agree. (02)
Allow me to sketch what would would then happen. (03)
First, we would have to decide what the right set
of "levels" should be. Should there be 5 of them,
as you suggest? Or 3, as Cyc has chosen? Four?
Twelve? Perhaps the whole idea of a total order
is wrong: can the levels be *partially* ordered?
(There's a PhD thesis lurking there.) After
interminable debate, most of it informed more by
raw intuition than anything substantive, we
might, possibly, more from exhaustion than
anything else, decide to settle on some smallish
number. Say five. That decision could easily take
six months of weekly debate, assuming leadership
by a strong chairman. If left uncontrolled, the
debates could be endless. (04)
Then, we will have to discuss what these levels
really mean. Some will urge that they should be
understood operationally: others will insist
that, being semantic distinctions at root, they
must be defined semantically or at the very least
connected to the semantics. The net effect of
this particular kind of debate is often that the
few people in the community who are competent in
formal semantics can hold the others hostage, if
they choose to be obstructive, or can block all
progress, if they quarrel amongst themselves (I
have been in WGs in which both scenarios have
played out). But suppose that we manage to
somehow resolve these issues in a way that all
parties can just accept, with various kinds of
gritted teeth. This will take another 6 months at
least, and will probably be like a volcano
waiting to erupt again at the slightest
subsequent change or debate. (05)
Then software must be written which actually uses
this stuff, involving discussions among a
community of implementers about the practical
strategies and permissible interpretations of the
new standards. This will take another year or so. (06)
Then every ontology writer has a huge extra task
to perform. It is no longer enough to simply
axiomatize one's knowledge - itself a task so
daunting and tricky to get right that it
represents the single most solid barrier to wide
deployment of the technology - but one must also
provide an elaborate meta-theory of it. People
will not do this consistently, which will create
new problems of interchange and interoperation.
And it solves none of the old problems, which we
already have. (07)
Finally, and worst of all, there are clear
pressures to not use this markup in the way
intended (even if we can somehow get this clear),
since if an honest person publishes an ontology
which is carefully and accurately meta-marked-up,
their work can be nullified (on an open network)
by someone else who adopts an elementary 'cuckoo'
strategy of publishing a rival ontology in which
the meta-markup declares everything to have level
1, thereby causing truth maintenance engines to
destroy other ontological content with which it
is inconsistent. The possibilities for hostile
takeovers, moles and 'semantic viruses' are
clear. In fact, they are so clear that I venture
to predict that if this ever were deployed, it
would immediately be hidden behind security
firewalls so opaque that it would be invisible. (08)
Pat (09)
> Following are some
>"levels of entrenchment" in descending order of strength:
>
> 1. Type hierarchy. The classical tree or partial ordering
> introduced by Aristotle and first drawn by (or attributed
> to) Porphyry. It's useful in every field, it's not going
> away, and we should recognize it as the minimal requirement
> for an ontology.
>
> 2. Necessary distinctions. The differentiae that split any
> type into two or more subtypes. If the split is binary
> (A or not-A), then it is both necessary and sufficient for
> distinguishing the two subtypes from one another, but the
> conditions for characterizing the supertype might not be
> necessary and sufficient.
>
> 3. Constraints. Additional statements that characterize the
> types or the interactions of entities of various types.
> The constraints are necessary relative to the ordinary
> facts in level #4, but they might not be considered
> defining characteristics.
>
> 4. Ordinary facts. Ground-level assertions that must be
> consistent with statements at the above levels, but they
> may violate defaults at level 5.
>
> 5. Defaults and probabilities. Statements that are usually
> true of entities of a given type or types, but they are
> at the bottom of the entrenchment pole. A probable
> statement is a default with an associated value that
> indicates its likelihood or frequency of occurrence,
> given the occurrence of some other condition.
>
>Systems of entrenchment levels along such lines are widely
>used and should be supported. Cyc, for example, has 3 levels:
>True, true by default, unknown (and the negations -- false
>by default and false). But I think that Lenat would agree
>that a privileged level should be added for some of the
>axioms, especially ones that define the type hierarchy.
>
>A declaration of which level a particular statement belongs to
>would not be part of the first-order theory, but it would be
>a metalevel statement that should definitely be considered
>part of the ontology.
>
>John
>
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