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# Nonmonotonic reasoning-situation by situation

We use circumscription to minimize the events that occur in a situation, the fluents that might prevent an event from having its standard effect, and the changes in fluents. In contrast to the formalism of [McC86] which minimized predicates over all the arguments, we minimize for each successive situation separately. However, in doing this minimization in s we take as fixed the Holds(f,s) sentences and the sentences inferred from the effects of the event that led to the situation. We are giving up the possibility of trading and abnormality in one situation for an abnormality in another. Doing the nonmonotonic reasoning in situations successively corresponds to the way people predict the consequences of sequences of actions and events. It seems to give the same conclusions as Yoav Shoham's chronological minimization [Sho88] but is computationally more straightforward. Like chronological minimization, it avoids the Yale shooting problem and its friends.

However, we advocate this only for projection problems, i.e. reasoning about the future from information about the past. The method is not appropriate for the stolen car scenario in which one has to reason from an assertion (that the car is missing) about a later situation.

With the present formalism, the person or agent setting up the problem must know that projection forward in time is appropriate. It would be better if this were a consequence of the formalized facts.

Now let's consider circumscribing at each situation separately. The simplest case is when we have a predicate Foo(x,y,s).

We write the axioms

Then the circumscription of Foo(x,y,s) takes the form

Here vars stands for a list of the entities being varied as Foo is minimized.

This spells out to

Call this formula Circ(Axiom;Foo;vars;s). This is the notation of [Lif94] with the addition of the argument s to say that s is kept fixed.

The general frame axioms are

for propositional fluents and

for general fluents.

Suppose we allow complex fluents, say when p and q are propositional fluents. We then need an axiom

Similar axioms are required for the other propositional functions of fluents and for the compositions of non-propositional fluents.

[This leads to difficulties when we want to delimit what changes, since there are arbitrarily complex compositions of fluents. We'll confine ourselves to elementary fluents for now by not putting compositions in the language.]

In these circumscriptions we also minimize Holds.

This tolerates elaborations like

If Holds(Weak,s) isn't asserted, Move(x,y) will not be prevented.

Lin and Shoham, [LS95] consider a theory of action to be provably correct if doing the nonmonotonic reasoning results in a complete nonmonotonic theory of the action. This seems like a worthy goal, but I don't know if the present theory achieves it.

Next: Actions and other events Up: ACTIONS AND OTHER EVENTS Previous: The blocks world

John McCarthy
Fri Feb 8 17:29:20 PST 2002