Reasoning about Events--Especially Actions

 

Reasoning about actions has been a major AI activity, but this paper will not discuss my or other people's current approaches, concentrating instead on the long range problem of reaching human level capability. We regard actions as particular kinds of events and therefore propose subsuming reasoning about actions under the heading of reasoning about events.

Most reasoning about events has concerned determining the effects of an explicitly given sequence of actions by a single actor. Within this framework various problems have been studied.

• The frame problem concerns not having to state what does not change when an event occurs.
• The qualification problem concerns not having to state all the preconditions of an action or other event. The point is both to limit the set of preconditions and also to jump to the conclusion that unstated others will be fulfilled unless there is evidence to the contrary. For example, wearing clothes is a precondition for airline travel, but the travel agent will not tell his customer to be sure and wear clothes.
• The ramification problem concerns how to treat side-effects of events other than the principal effect mentioned in the event description.

Each of these involves elaboration tolerance, e.g. adding descriptions of the effects of additional events without having to change the descriptions of the events already described. When I wrote about applications of circumscription to formalizing common sense [McCarthy, 1986], I hoped that a simple abnormality theory would suffice for all of them. That didn't work out when I tried it, but I still think a common nonmonotonic reasoning mechanism will work. Tom Costello's draft ``The Expressive Power of Circumscripttion'' argues that simple abnormality theories have the same expressive power as more elaborate nonmonotonic formalisms that have been proposed.

Human level intelligence requires reasoning about strategies of action, i.e. action programs. It also requires considering multiple actors and also concurrent events and continuous events. Clearly we have a long way to go.

Some of these points are discussed in a draft on narrative [McCarthy, 1995].