RE: Perdurantist planning problems (was: RE: RE: SUO: Re: Ballot Comment)
Dear Pat,
See comments below.
Matthew West
Principal Consultant
Shell Information Technology International Limited
Shell Centre, London SE1 7NA, United Kingdom
Tel: +44 20 7934 4490 Other Tel: +44 7796 336538
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> -----Original Message-----
> From: pat hayes [mailto:phayes@ai.uwf.edu]
> Sent: 01 September 2001 00:18
> To: standard-upper-ontology@ieee.org
> Cc: West, Matthew R SITI-GREA-UK; apease@ks.teknowledge.com
> Subject: Perdurantist planning problems (was: RE: RE: SUO: Re: Ballot
> Comment)
>
>
> Now that Adam and Matthew seem to be converging on a mutual
> acceptance of a spatiotemporal ontology which descibes change in
> terms of atemporal assertions about temporal parts, and since there
> seems to be no chorus of protest from the endurantists at this
> flagrant disregard for their philosophical scruples (Chris Menzel?
> Mike Gruninger? Are you there?), allow me to mention one severely
> practical problem with this approach.
MW: Well I am trying to persuade Adam to include an additional 4D
view, not to replace the 3D view with a 4D view, though I agree Adam
is heading in that direction at the moment. Anyway ...
>
> There is a well-entrenched tradition of 'action planning' in AI which
> thinks of a dynamic world as moving through states under the control
> of 'actions', and does planning by proving that states exist which
> satisfy certain properties, extracting the sequence of actions to
> achieve the goal by examining the proof. The oldest form of this is
> planning in the situation calculus, but basically the same idea has
> been used in a number of different settings.
MW: Well that sounds familiar - do you mean things like Petri nets? If
not could you give some references please?
> What all these have in
> common is an assumption that it makes sense to reason about changes
> in terms of a tree (or directed graph) of states and
> state-transitions, where the states in the immediate 'future' of a
> state are the alternative possibilities for the next state. Notice
> that this picture combines two rather different modal ideas, in that
> one 'dimension' (following paths in the graph) corresponds to time,
> while the other (the fanouts from each node) to possibility. This is
> very difficult to reconcile with the perdurantist (4-d) ontological
> language, since there is no single spatiotemporal 'envelope' which
> contains all the various possible futures of a given state.
MW: Oh. I hadn't noticed, and planning is one of the things that is
important.
MW: Lets get a few preliminaries out of the way. When you say "state"
and "state-transition" you are not refering to specfic spatio-temporal
extents, but classes of spatio-temporal extent, and the relations between
them about possibility are what I would describe as classes of relation
that determine some rules of what is allowed between members of these
classes. Right?
>
> The situation is not impossible, since one can think of the planning
> process as searching through a space of 'expanding' alternative
> space-time bubbles, but it gets woefully complex to describe, in
> painful contrast to the elegant style of function nesting which
> arises from the older ontological frameworks;
MW: My ignorance is showing here. Could you explain, and perhaps give
an example of what you mean by "function nesting".
> and this inelegance has
> some drastic and possibly fatal computational consequences, since one
> cannot rely on unification to automatically keep track of the
> spatiotemporal-part relationships which arise here in the same way
> that it automatically creates action-sequence terms; the 'fit' of the
> tree of possible futures with the tree-structure of action terms is a
> very valuable byproduct of logical reasoning; for many people, in
> fact, the main product.
MW: OK now I really need examples.
>
> BTW, this is why I long ago gave up on what was otherwise the very
> promising 4-d approach to naive physics which I developed in my old
> 'liquids' paper. If what one wants to do is simply describe a
> particular timeline, then perdurantism has many advantages. But to
> reason about a complex system of partially overlapping possibilities,
> it seems to give rise to many ugly and intractable problems, and the
> elegance of the endurantist simplicity of the situation calculus and
> its variations becomes very appealing by contrast.
MW: Well I agree that reasoning about the future is difficult. But I
hadn't noticed it being easier from an endurantist viewpoint. In fact
some things became clearer for us when we moved to a perdurantist
view point, but maybe some different things than you have been looking
at.
>
> Pat Hayes.
> ---------------------------------------------------------------------
> (650)859 6569 w
> (650)494 3973 h (until September)
> phayes@ai.uwf.edu
> http://www.coginst.uwf.edu/~phayes
>