Perdurantist planning problems (was: RE: RE: SUO: Re: BallotComment)
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.
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. 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.
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; 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.
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.
Pat Hayes.
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