14.1. State in Perspectives
Perspectives describes part of the world in terms of contexts, roles and their properties. But we do not consider such a description to be universally valid or useful. Hence we limit access to its parts by perspectives of user roles. Only those participating in a context have access to it and the tacit assumption is that they will know how to interpret its representations (I use description and representation as synonyms in this text).
The Perspectives universe is not God-given. Rather, the participants build it piece by piece. There are at least two reasons to do so: to cover larger part of the world by a description and because the world itself changes, so the description has to follow.
Anyhow, it is useful to think about the state of a Perspectives representation. This state consists, obviously, of the state of its parts. The state of a context instance is determined by its role instances; that of a role instance by its filler(s) and the roles it fills; and the values of a property type for a particular role instance could be seen as property state. However, for practical reasons we collapse role- and property state together into role state.
Given a particular set of Perspectives types (context-, role- and property types), the number of states a certain description of the world in terms of those types can assume can be very large indeed (if the number of role instances is not limited, the number of states is infinite). Therefore, rather than thinking in terms of individual states, it is useful to think in terms of state collections (some authors speak of micro- and macrostates).
So when is a state member of a state collection? It turns out that for role state, this is governed by a proposition; a sentence in propositional logic. Or, in less fanciful words: when a set of equalities or comparisons of properties, combined with ‘and’ and ‘or’, is true. In Perspectives terms, this is a property query with a Boolean value.
For context state we use a sentence in predicate logic. That is, we apply quantification, such as ‘for all’ and ‘exists’ to roles and contexts. Nevertheless, in Perspectives terms, this is again a property query.
Now from this point on we will use ‘state’ instead of the more correct ‘state collection’ and we will say that it is defined by a state query. We also associate a state label with the query and use it to identify that state.