# Parts and lack of them

An important aspect of folk physics is the reasoning about parts / whole relationships. These are represented by relationships in Xapagy, the basic part relationship being provided by the verb |contains|.

We can say that Billy has a head by:

A "Billy" / is-a / man.
"Billy" / contains / a head.

The fact that most humans have a head is part of the autobiographical information of the agent. This needs to be put in contrast to default logic or frame systems, where there is a logical description of the structure.

In most stories, we do not need to enumerate the body parts of the agents. With a different autobiography, the Xapagy agent will maintain them as a headless shadow, thus when we refer to |the head — of — Billy|, the instance will be generated automatically.

{\em {\bf Future work:} this create on demand by reference is present in Xapagy but it is largely untested and used very rarely. It should get more use in larger autobiographies.}

The next question is how do we represent the fact that a certain part is missing, despite the fact that it would be predicted by the autobiography? The fact that it had not been enumerated in the current scene is not sufficient to indicate its absence.

Let us consider that we want to talk about the headless horseman. We need to represent the absence of the head {\em explicitly}. Simply not mentioning the existence of the head is not enough, as we have seen, for most humans the stories do not explicitly mention the existence of the head, yet we usually assume its presence.

This is accomplished by explicitly creating an instance for the head which has the |inexistent| attribute. This instance will be put in the appropriate part relation with the human. This would allow it to be explicitly shadowed by other headless humans from the autobiography:

A "HeadlessHorseman" / is-a / man.