RE: SUO: On the (existence of the) supreme supertype
Phil, I won't say anything else about what we call T, but you do
raise an important issue, which can be summarized as the question of
whether existence should be a predicate. There is a potential
mismatch between 'exists' meaning actual existence in the real world
and 'exists' used as a quantifier in the logic. If we want our logic
to be able to reason about circumstances which might never actually
obtain (as is often done in planning. by considering alternative
possible futures, then deciding to perform actions which make some of
them impossible; and as is also done in counterfactual reasoning),
then we have to allow into the domain of quantification things which
do not in fact exist in the first sense, and also be able to
contemplate things which do exist having properties which they never
in fact have. So just saying
(exists (?x)(= ?x a))
wouldnt be enough to assert that the thing called 'a' *really*
exists; to do that one would need to say something more about this
thing called 'a'. Exactly what more is an engineering question which
we need to address.
A common strategy is to introduce 'imaginary entities' or
'possibilia' as a distinct category in the classification heirarchy,
but I would argue that this is a mistake, on several grounds. First,
any kind of thing might be possible: one can have possible humans,
possible events, possible worlds, etc.., so as a category this cuts
across the entire classification spectrum. Second, it suggests that
possible-but-unreal things are somehow another category of real
things, much as birds are a kind of organism. This gives rise to many
well-known concpetual puzzles and paradoxes, all arising from a basic
misunderstanding. (Quine in a famous passage points out the error in
going from 'it is possible that someone could have been standing in
the empty doorway' to 'there is a possible person standing in the
empty doorway', by asking *how many* possible but non-actual people
every empty doorway has to contain?) And third, it masks the
important point that these non-actual things almost always seem to be
'inside' non-actual worlds, ie we talk about an alternative set of
circumstances or alternative possible worlds (in which, say, I had a
brother who I don't in fact have), rather than putting the possible
but non-actual things into this real world. And there is a good
reason for this: if the possible-but-non-actual things were in fact
actual, they would have real effects on the rest of the world. One
cannot just insert a thing into the world and expect everything else
to go on just as before; so one is forced into thinking about a wider
set of circumstances than just the immediate surroundings of the
imaginary thing. As we all know, one dead sparrow might have all
kinds of repercussions, so it is usually safest to provide an
alternative world for the sparrow to die in, and see what happens
there.
I think that a general-purpose SUO needs to allow non-actual entities
- it cannot presume that the existential quantifier means existence
in the real world - but that just allowing them casually, as it were,
without some attention being given to their effects, is a well-known
recipe for conceptual muddle. I would suggest that any nonreal thing
needs to be thought of as enclosed within a non-real causal bubble
which is large enough to provide a causal 'horizon' insulating any
effects of the unreal thing from influencing any events in the real
world, or vice versa. The safest such bubble would be an entire
possible world, but we might allow somewhat smaller sets of
circumstances or 4-d enclosing space-times to play the same role.
Then the way to introduce 'possibilia' into the ontology is to
introduce these 'bubbles', and allow 'IsReal' as a predicate on them
(which asserts that that set of circumstances does in fact obtain in
the real world somewhere/when). Then a possible-but-not-real person
(eg my imaginary brother) is simply a normal person who has the
misfortune to only be enclosed inside nonreal bubbles. This overall
picture has no need for a special category of 'possibilia',
therefore, and it is meaningless to even talk about
possible-but-unreal things inside real worlds.
Pat Hayes
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