PopBeast now has to start planning a sequence of actions to achieve
this. The first step is to find out exactly what ``open'' means. This is
done by looking up
opens in a table which gives its meaning in
terms of how it changes the world. In Eden, once you have opened a door,
it disappears, i.e. the square in which it stood becomes empty. So the
meaning can be expressed as ``The effect of opening a door is that at
the start of the activity there is a door in some square S, and at the
end of the activity there isn't''. In PopBeast's memory, this is stored
verb( opens(D), [ square(D,S),door(D) ], [ clear(S) ] ).Here,
verbis a proposition which expresses information about other propositions. The first argument is the predicate which we used to represent ``opens'', the second is a statement about the start of the activity of opening, and the third is a statement about the end of the activity.
clear(S)is how PopBeast represents the fact that square S is empty. If we can fill in the current values for
S, we will have a description of our current goal - to convert a state in which square
Scontained a door to one in which it doesn't.
Note: as with world-models, not everyone believes that successful artificial intelligences will contain explicit representations of their goals. This view is descended from the idea that goals, beliefs and other ``folk-psychology'' constructs are a pre-scientific means we have developed for describing our own and other minds; they no more describe what really goes on than astrology describes how the cosmos operates. There are therefore no explicit representations of such things in human and animal minds: trying to find them is bad cognitive science. Furthermore, trying to build such representations into artificial minds is bad engineering. These views are put in Taking Eliminative Materialism Seriously: A methodology for Autonomous Systems Research by Tim Smithers, from Towards a Practice of Autonomous Systems, edited by Varela and Bourgine (MIT 1992: PSY KH:V 042; RSL M92.C00938); and in An Emerging Paradigm in Robot Architecture by Malcolm, Smithers and Hallam (AI box photocopy M149).
PopBeast does represent such entities - for teaching purposes, to illustrate classical AI. So going back to verbs, other ones could be expressed in the same way. The meaning of ``go to object X'' is stored as
verb( go(X), [ square(X,S)], [ square(X,S),square(me,S) ] ).This expresses the fact that at the start of going, X is in a certain square called S, and at the end of going, PopBeast -
me- is also in S.