Today

Today, the ontologies discussion continues, with questions by Michael Gelfond and Luis Pereira.

Debates

Michael Gelfond:

I would like to better understand the following comment by Hector Geffner:

I believe they (models of actions in AI) are all monotonic in the set of observations. In other words, if they predict  F  at time  i , nothing that they observe is going to affect that prediction.

If I understood Hector correctly, the following may be a counter example. Consider the following domain description D0 in the language  L  from [j-jlp-31-201].

The language of D0 contains names for two actions,  A  and  B , and two fluents,  F  and  P . D0 consists of two causal laws and two statements describing the initial situation  S0 :

	A causes F if P.
	B causes neg(P).
	true_at(P, S0).
	true_at(neg(F), S0).

The first statement says that  F  will be true after execution of  A  in any situation in which  P  is true. The third one means that  P  is true in the initial situation  S0 .  neg(P)  stands for negation of  P .

(Domain descriptions in  L  allow two other types of statements:  occurs(A, S)  - action  A  occurred at situation  S , and  S1 < S2 . We use them later)

Here we are interested in queries of the type

holds(F,  [A1, ...An] )

which can be read as ``If sequence  A1...An  were executed starting in the current situation then fluent  F  would be true afterwards''. This seems to correspond to Hector's prediction of  F  at time  i . We can also ask about occurrences of actions, truth of fluents in actual situations, etc).

The entailment relation on  L  between domain descriptions and queries formalizes the following informal assumptions:

• changes in the values of fluents can only be caused by execution of actions;

• there are no actions except those from the language of the domain description;

• there are no effects of actions except those specified by the causal laws of the domain;

• reasoner assumes that if there is no reason to believe that action  A  occured in situation  S  then it did not (In other words he normally observes all occurrences of actions).

As expected, we have that

D0 entails holds(F,  [A] )

Now assume that the agent observed (or performed)  B . This will be recorded in his description of the domain. New domain description D1 is D0 plus the statement

occurs(B, S0).

Now we have that D1 entails  neg(holds(F,  [A] )) . It seems to me that the observation changed the prediction.

The second example shows how observations can change beliefs about the past. Consider a domain description D3

	A causes neg(F).
	F at S0.

This description entails  neg(occurs(A, S0)) . Now the reasoner observed that in some situation  S1 ,  F  is false. This is recorded by adding to D3

	S0 < S1
	neg(F) at S1.

The new description entails  occurs(A, S0) . Again, observations changed the belief (this time about the past).

Hector, is this really a counter example or you meant something else?

Reference.

C. Baral, M. Gelfond, A. Provetti, ``Representing Actions: Laws, Observations and Hypotheses'', Journal of Logic Programming, vol. 31, Num. 1,2 and 3, pp. 201-245, 1997.

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Luís Moniz Pereira:

Dear Erik,

I noticed in the discussion that you said:

From the point of view of diagnostic reasoning these are familiar problems, but I can't think of any work in mainstream actions and change that has addressed nonmonotonicity with respect to observations in a serious way.

I have tackled the issue of nonmonotonicty with respect to observations. Cf my home page, the AAAI-96, ECAI-96, AIMSA-96, LPKR97, JANCL97, AI&MATH98 papers. Using a LP approach I perform abuction to explain observations. The abductive explanations may be: non-inertiality of some fluent with respect to some action; occurrence of some erstwhile unsuspected foreign concurrent action along with some action of mine; or opting for a definite initial state of the world up till then given only by a disjunction of possibilities.

You're right, the techniques I and my co-author, Renwei Li, use were first developed by me and others in the context of diagnosis using LP! In fact we haven't yet used them all up yet in actions. For a view of LP and diagnosis, as well as representing actions in LP, see our book [mb-Alferes-96].

Best, Luís

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