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Re: [ontolog-forum] Thing and Class

To: "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Pat Hayes <phayes@xxxxxxx>
Date: Fri, 12 Sep 2008 16:52:05 -0500
Message-id: <8147D344-92FF-4734-AF20-2B16D57884D9@xxxxxxx>

On Sep 12, 2008, at 1:01 PM, Chris Partridge wrote:    (01)

>> Indeed. And of course, change occurs in the real world and is
>> described by both 3-d and 4-d ontologies, but in different ways. This
>> is so obvious that nobody felt any need to say it.
>
> Some of this discussion of change and 3d and 4d seems to be covering  
> (very)
> old ground.    (02)

Well, thats certainly true. The philosophy goes back to the late  
Bronze age, probably, but even as part of ontology engineering we were  
having these discussions over a decade ago.    (03)

> It is my impression that in the philosophical tradition that in time  
> gave
> rise to the short names 3d and 4d (I think these are only decades  
> old), it
> is usual to say that the 4d view denies the existence of change.    (04)

Maybe, but its still extremely misleading. I've been a 4-d guy since  
at least 1973, and I've never thought of it as denying change. In  
fact, I felt forced into it in order to be able to describe such  
changes as water pouring out of a jug onto a tabletop, or Lac Leman  
changing color from brown to blue in one weekend.    (05)

> See. For example, http://plato.stanford.edu/entries/change/
> Where it has the nice comment:
>
> It is on the face of it extremely implausible to deny change, but  
> extreme
> implausibility has not always deterred philosophers. The Eleatics  
> (C5th
> BCE), particularly Parmenides, appear to have been the first to do so.
>
> More recently - McTaggart, J.E., 1908, "The Unreality of Time," - with
> Series A and B. And then a plethora of authors in the last few  
> decades.    (06)

Last few decades? Citations? The most comprehensive account of  
fourdimensionalism is probably the book by Theodore Sider, cited by  
Matthew in an earlier message in this thread: and it certainly does  
not deny the existence of change.    (07)

BTW, there is a really good review/summary of that book here, which  
everyone should read before commenting further:    (08)

http://ndpr.nd.edu/review.cfm?id=1135    (09)

>
> As this and other entries in the site explain, the issue is how we  
> explain
> the changes we see - and 3d and 4d are alternatives.
>
> see also http://plato.stanford.edu/entries/temporal-parts/ - Change]
>
> Take the definition of an ontology as the set of things that exist,  
> and we
> start compiling our list ....
>
> Then in the traditional Aristotle 3d view, there are objects that are
> changes. For example, my movement from A to B.    (010)

That is a 4-d entity, not a 3-d one. There is no room for things  
called movements in a 3-d ontology. There are places where movements  
happen, and there can be movement-functions or movement-actions  
considered as state-functions, but not genuine physical space-time- 
occupying movement-things.    (011)

> In the 4d view, my movement from A to B consists in my being 'at' A  
> at time
> t1 and 'at' B at time t2 (Russell characterised this as an at-at
> description).    (012)

Wrong! That is a 3-d state-based or proposition-based view (Im not  
sure which from your description). The ability to treat a movement as  
a genuine first-class thing, as opposed to a pair of static states, is  
exactly one of the advantages of a 4-d as opposed to a 3-d ontology.    (013)

>
>
> One can of course, and philosophers like to do it, take elements of  
> the 3d
> and 4d view and concoct gerrymandered positions, where something  
> that looks
> 3d has 4d elements.
>
> One may also take issue with the basic claim about change. But from a
> historical point of view, I think there is a good (overwhelming?)  
> case for
> saying that the 4d position has been characterised for some time as  
> denying
> the existence of change (in the sense described above).
>
> One can add the gloss that 4d-ists (Parmenideans) did not really  
> mean that
> change could not be described in a 4d view - but it seems to me odd  
> to claim
> that 4d has not been characterised as denying the existence of  
> change, and
> also to miss an insight into how the position arose, if not into  
> what it is.    (014)

Maybe we are talking about different things. I have no idea how  
Parmenides got into the act, but by 4-d I mean to refer to 'four- 
dimensionalism' in the sense often called 'eternalism', which is  
simply the view that things exist extended in time: put another way,  
that physical things have temporal parts. It is opposed to the view  
(embedded into several current high-level ontologies such as BOF and  
DOLCE), which allows for 'continuants', which are wholly 3-d but  
'continue' through time without actually occupying it. I'm pretty sure  
that is what Matthew is talking about, also.    (015)

Pat    (016)

PS.:    (017)

> BTW Pat's comment copied in at the top "of course, change occurs in  
> the real
> world" would seem to give precedence to the 3d view that changes  
> should be
> on our ontology list. But I have taken it out of context.    (018)

OF COURSE changes should be on our ontology list. The 4-d world  
consists of very little else.    (019)


>
>
> Chris
>
>> -----Original Message-----
>> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx [mailto:ontolog-forum-
>> bounces@xxxxxxxxxxxxxxxx] On Behalf Of Pat Hayes
>> Sent: 12 September 2008 18:16
>> To: [ontolog-forum]
>> Subject: Re: [ontolog-forum] Thing and Class
>>
>>
>> On Sep 12, 2008, at 1:11 AM, John F. Sowa wrote:
>>
>>> Dear Matthew and Pat,
>>>
>>> MW> The real difference is that 3D sees that what exists now is all
>>>> that exists, whilst 4D sees the past and the future as part of
>>>> what exists as well as the present. This is what it means to stand
>>>> outside time.
>>>
>>> I agree with that description, but you seemed to suggest that the
>>> notion of change does not exist in a 4-d view, but I think that
>>> we were using different definitions of 'change'.
>>
>> Indeed. And of course, change occurs in the real world and is
>> described by both 3-d and 4-d ontologies, but in different ways. This
>> is so obvious that nobody felt any need to say it.
>>
>>>
>>>
>>> MW>> all spatio-temporal extents exist (at all times, but
>>>>> strictly independent of time).
>>>
>>> PH> Agreed, and a nice analysis. Putting the same point in logical
>>>> terms, the universe of discourse shouldn't be in a state of flux,
>>>
>>> I have no quarrel with that, but it has nothing to do with the
>>> definition of the concept of change.
>>
>> BUt that, unlike the 'concept of change' , is what the thread was
>> about until you introduced this irrelevant aside. I think you can  
>> take
>> it that most of us don't need elementary calculus explained to us
>> again, John.
>>
>>> According to the most common
>>> definition, if time slices at t=0 and t=1 are identical, there is
>>> no change.
>>>
>>> Another way to say it:  if the partial derivative with respect
>>> to the time coordinate is 0, there is no change; otherwise, there
>>> is change in that region of space-time.  The existence of change
>>> does not imply that the global 4-d universe is in flux.  It just
>>> means that there is some region in the universe where the derivative
>>> with respect to time is not zero.
>>
>> Quite.
>>
>>>
>>>
>>> MW>> And interestingly, I again use possible worlds as an  
>>> alternative
>>>>> to modal logic. Not that I object to others using modal logic, but
>>>>> I do not see that I am obliged inevitably to do so.
>>>
>>> PH> Again, I agree that this is the best approach. I think this is
>>>> widely accepted, by the way: John McCarthy made the same point many
>>>> years ago :
>>>> http://www-formal.stanford.edu/jmc/modality/modality.html
>>>
>>> I believe that what John McC, Matthew, and Pat are recommending is
>>> very close to Dunn's semantics for modal logic.
>>
>> Not McCarthy or me, I am quite sure; and given his stance on 4-d
>> extensionality, I doubt if Matthew is either.
>>
>>>
>>>
>>> Most AI work with "possible worlds" is actually based on metalevel
>>> reasoning about sets of propositions that describe those worlds,
>>> not with the worlds themselves.
>>
>> Wrong. It is based on - it actually uses - inferences made in a  
>> theory
>> which refers directly to the worlds. Any planner which has an  
>> explicit
>> notion of time-interval or time-point or 'situation' (as in sit.
>> calc., not situation theory) or world-state or context, is reasoning
>> in a theory whose semantics is more Kripkean than Dunnian. (For an
>> early exposition of the relationship of situation calculus to Krikpe,
>> see section 4 of McCarthy & Hayes 1969.) None of this is based on
>> METAlevel reasoning about sets of propositions. Of course, like all
>> reasoning, it is PERFORMED by sets of sentences, but it is not ABOUT
>> them.
>>
>>> Starting with any Kripke model
>>> K=(W,R,Phi), where W is the set of words, R is the accessibility
>>> relation among worlds, and Phi is the evaluation function, those
>>> sets can be derived:
>>>
>>> 1. For each word w in W, define the facts of w as the set of all
>>>    propositions p that are true in w:  {p | Phi(w,p) = True}.
>>>
>>> 2. Define the laws of w as the set of all propositions p that are
>>>    necessarily true; i.e., p is true in all worlds accessible from  
>>> w.
>>>
>>> 3. Define the accessibility relation R(w, w') as True iff every
>>>    proposition p that is necessarily true in w is also true in w'.
>>>
>>> This construction replaces every world in a Kripke model with a set
>>> of laws and facts in a Dunn-style model.  Any theorem that can be
>>> proved about a Kripke model is also true of the corresponding Dunn
>>> model.  But Dunn's version is more *usable* because it makes the
>>> laws and facts available for further analysis and manipulation.
>>
>> Nonsense. Not only is Dunn's version not more usable, it is not in
>> fact used. The laws and facts are stated explicitly as sentences in
>> theories using Kripke-style semantics. Check out any of the hundreds
>> of papers on planning using a situation-calculus style of
>> representation.
>>
>>> PH> John's way follows Dunn's theory and is  based on intensional
>>>> descriptions.  The far more commonly used view uses Kripke's
>>>> possible-worlds account of modalities. Kripke's is widely accepted
>>>> as the standard, and certainly gives a more usable semantics...
>>>
>>> Not true.  Nobody actually implements "possible worlds"
>>
>> I never said they did. They implement systems which reason, using
>> formalisms which refer to possible worlds. Remember, we are talking
>> here about SEMANTICS, not implementation.
>>
>>> .  What they
>>> implement and reason with and about are sets of statements of the
>>> laws and facts of those worlds.
>>
>> Yes, exactly. And the question is, what is the appropriate SEMANTICS
>> for those statements? Answer: they REFER TO possible worlds. They do
>> not refer to sets of sentences; they are not in a METAtheory.
>>
>>> Since the above construction can
>>> map any Kripke model into such sets, most people who implement such
>>> systems pay lip service to Kripke's version, but they actually use
>>> something that is much closer to Dunn's version.
>>
>> They - and here I speak as one of them - use Kripke-style semantics
>> when doing semantic analysis. They do not, as a broad rule, use Dunn-
>> style semantics. In fact, I do not know of any significant body of
>> work in AI planning based on Dunn-style semantics. I believe you are
>> the only writer who argues for Dunn's model in this context.
>>
>> Pat
>>
>>>
>>>
>>> For further discussion of these and related issues, see
>>>
>>>   http://www.jfsowa.com/pubs/laws.htm
>>>   Laws, Facts, and Contexts
>>>
>>>   http://www.jfsowa.com/pubs/worlds.pdf
>>>   Worlds, Models, and Descriptions
>>>
>>> John
>>>
>>>
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IHMC                                     (850)434 8903 or (650)494 3973
40 South Alcaniz St.           (850)202 4416   office
Pensacola                            (850)202 4440   fax
FL 32502                              (850)291 0667   mobile
phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes    (021)






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