Ontologies for actions and change
Pat Hayes:
Responses to Judea Pearl and Erik Sandewall in ENRAC 26.1 (issue 98009):
Responses to Judea (all quotations from his message 26.1):
On Actions vs Observations, or on Pat Hayes' reply to Geffner,
Poole and me:
| ... the cleavage between the culture that Hector, David and I
represent and the one represented by Pat has gotten so DEEP that we
are not even sure we are talking about the same thing.
| Indeed. I started to write a blow-by-blow response, but Judea and I trying
to out-flame each other is reminiscent of a Godzilla movie, so I will
ignore all the rhetoric and just try to summarise what seem to me to be the
essential points of difference. First however, to clear up one basic
misunderstanding:
| Pat does not think the "distinction between sensing and acting is
either necessary or even ultimately coherent". For him, observing a
surprising fact evokes the same chain of reasoning as establishing
that fact by external act. In both cases, so claims Pat, the world
is changing, because a world is none other but one's beliefs about
the world, and these do change indeed in both cases.
| No, no. Of course the world cannot be identified with one's beliefs about it.
This distinction is central to my view of knowledge representation. The
world is changing because that is what worlds do, by and large; certainly
my own 'common-sense' world keeps changing all the time. If there were no
change, our ordinary notions of time and tense would be incoherent. Again,
this seems so obvious that I am sure nobody can disagree with it, so
Judea and I must be using the word 'change' differently, and indeed he
seems to have in mind something like a change of mind about the causal
structure of the world. I'd call this a change of belief rather than a
change in the world:
| ... there
is no need to invoke such dramatic phrases as "changing worlds" for things
that we know how to handle by standard inference methods. (e.g., adding
a proposition "light on" to the system and let a classical theorem
prover draw the consequences. [and if our current beliefs contain
"light off" then the contradiction can be handled by either temporal
precedence or minimal-change belief-revision]).
| As an aside here, I am far less sanguine about the ability of 'standard'
inference methods (whatever they are) to routinely draw consequences about
a causal world, and the general usefulness of such obviously false
'principles' as temporal preference or change-minimisation. (Judea seems to
think that all the interesting representational work is already done. I
don't know what makes him so optimistic.) But in any case, changes in the
world have nothing to do with inference methods. Oddly enough, Judea here
seems to be suffering from the same mistake he accuses me of, by confusing
the world with our models or descriptions of it.
| [we need to decide]... when a change is considered a "world-change" and
when it is merely a "belief change".
| Let me see, how can I possibly say this more clearly? A "world change" is a
change in the world, and a "belief change" is a change in belief. My point
is precisely that the time which the beliefs are about need bear no
simple relation to the time at which the beliefs are created. (This is
similar to the distinction in temporal databases between 'valid time' and
'transaction time'.)
| Question:
Is flicking the switch and seeing the light come on an example of
a changing world ?
| Yes.
| From a physical viewpoint, the world has obeyed Schrödinger's
equation before and after the action, so it has not changed at all.
| This remark seemed to come out of left field until I re-read Judea's
messages several more times, and then I suddenly got it. Judea's entire
framework is the Copenhagen interpretation of quantum mechanics. This is
where the otherwise mysterious talk of Schrödinger, the 'laws of
astrophysics', invariants, the need to establish 'boundaries', the sharp
distinction between actions and observations (which, in the Copenhagen
interpretation, must come from 'outside' the system boundaries, causing a
'collapse' of the wavefunction), the Niels-Bohrian talk of "all the
particles in the universe", and the odd idea that its observations which
change the 'world', all come from. It also may account for his doctrinal
confidence that his rather idiosyncratic perspective is accepted by most
philosophers (anyone who thinks that anything is accepted by most
philosophers obviously has never been in close association with
philosophers.) OK, if you think of our subject as likely to be informed by
quantum theory, then go ahead. Personally I disagree, for various reasons
which I wont take space up here enlarging on.
One last comment:
| ... we are not dealing here with physics or psychology. We are dealing
with various formal systems for modeling the world and our beliefs
about the world.
| To deny both physics and psychology, and then to immediately speak of
"world" and "beliefs about the world", seems to me to be at best careless.
Responses to Erik
In his message 26.1, referring to an earlier message by myself, he wrote:
| In the sitcalc (any variety), actions are changes in the world, not motor
commands. One plans by thinking about the changes, not by thinking about
the muscles one is going to use. Putting such a system into a robot
requires one to somehow connect these actions with motor controls, no
doubt, but they shouldnt be identified. (Murray and Ray, do y'all agree??)
|
| However, earlier in the same contribution Pat had written:
|
| ... Our peripheral systems often blend motor action and lowlevel perception
in tight feedback control loops, so that our bodies seem to 'move by
themselves', but these lowlevel controls are the result of more cognitive
decision-making (deciding to hit a tennis ball, say.)
|
| Pat, I can't make sense out of your position: at one point you seem to
argue that low-level and high-level descriptions of actions can't ever
be separated; at another point you seem to say that they are best
treated in complete separation.
| These two passages were in different contexts. I dont think that
observations and actions can be clearly separated, ultimately, basically
because observation processes often get so integrated into our actions that
the combination is best described together in terms of feedback and
control, as when we talk of 'moving carefully', or 'holding rigidly in
place'. I bet such mergings of observation and action will occur in robotic
applications pretty soon, in fact. But this is quite consistent with my
other point, since the action/observation combinations can be described and
reasoned about at a higher level than muscle signals, just as simpler
actions can; and this higher level - ie in terms of their relations to the
world rather than to anatomy - is usually the most appropriate for action
reasoning. In fact, in much of higher-level planning, they may be treated
similarly as 'actions', except that some actions require some observational
capability. For example, consider the fact that moving something carefully
depends on being able to see where it is, and inferring that careful
movement might require switching the light on (but a coarse job can be done
in the dark.)
| My own preference is to take both into account, but to be precise about
having two distinct levels of descriptions with distinct roles. In
particular, this allows for dealing both with an action as a
prescription for motor controls and as the expectation for what
state changes will be obtained as a result. It also allows one to relate
those two levels to each other.
| Well, I agree. I guess my original response (to David Poole) was a reaction
to his assumption that the 'muscle' case was the normal one. But maybe I
over-reacted. I'm quite happy with making a distinction like Erik's careful
one between "material" and "deliberative" levels, or something like that,
but he has misunderstood me slightly:
| ... The deliberative level is the one that Pat alludes to, where actions
are characterized by discrete properties at a small number of timepoints:
possibly only the beginning and the end of the action, possibly a few
more, possibly a sequence of partial world states at integer timepoints
(as in basic Features and fluents).
| I'm beginning to despair of being understood. Didnt this entire newsletter
discussion start with me saying that I don't want to endorse the
discrete-time-point way of describing change? Ive written extensively on
time-intervals, and the ontology I used for naive physics was based on
extended pieces of space-time ('histories') rather than anything pointlike.
The fact that someone with Erik's insight seems to be unable to think of
'high-level' as meaning anything other than instantaneous situations is a
vivid illustration of the problem Ive been complaining about: almost all
our thinking in this field is dominated by the archaic and awkward
oversimplifications embodied in the situation calculus.
David Poole wrote:
| What if I didn't know whether there was something on x when I tried
to pick it up? It seems that the only sensible interpretation of the
precondition is that if there was nothing on x and I carried out
pickup(x) , then the expected thing would happen. If something was on
x and I carried out pickup(x) then who knows what may happen. The
role of the precondition is that it is only sensible to attempt to
carry out the action when the preconditions hold.
| Pat Hayes answered:
| No no. This is all a description of an action. .....
| To which Erik Sandewall answered:
| I'll agree with David if the last sentence is changed to go "...it is
only sensible to attempt to carry out the action when the preconditions
are believed to hold". .....
Therefore, Pat, I don't understand what you are objecting against in
this respect.
| I have no objection to your rephrasing, but I dont think its a trivial
change: that's exactly what I was reacting to. The distinction between a
fact in the world and a belief in the planner is an important one when we
are having a discussion about action and observation.
| (And what does it have to do with the original need for nonmonotonic
logic?)
| Because when an observation contradicts a belief, we would seem to have
contradictory beliefs, unless our logic is nonmonotonic. This was the
original motivation for McCarthy's use of circumscription, to allow the
reasoner to use implications of the form
|
if Property(state) then Otherproperty(do(action, state))
| |
even when there could clearly be circumstances where, in fact, if the
action were taken, the conclusion might not be true (the block might not be
held if the grippers were oily). If we know ahead of time that actions
always come out the way we want them to, then we could write change axioms
which were guaranteed to be correct, and circumscription wouldnt be
necessary (just as it often isnt necessary when describing programs, where
it is possible to give a complete description of the relevant parts of
the 'world'-state - where 'world' here means the computer the program is
planned to run on, of course.)
Im puzzled that you need to ask this question. Isnt this textbook-level
stuff?
Pat Hayes
|