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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