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\section{Introduction}

This chapter is meant as a self-contained introduction to
the main themes and technical contributions of
situation theory.  This is a body of work
which is concerned with several foundational points
having to do with {\em information}.  We are interested
in modeling the entities which
bear information, and then in seeing how logic 
is related to them.


In writing this chapter, we had the conflicting goals
of presenting the existing literature on the subject,
and reorganizing it. 
As we see matters, a basic
problem in the existing literature 
is that it has not yet  produced a single unified
theory.  It is  more  a collection of technical
contributions, some quite sophisticated,
 that have not been put together.
However, we decided not to simply survey this material,
but rather
to  re-present situation theory, so that readers unfamiliar
with it could hope to learn what it is about. 
In effect, we reconstruct the basic ``ontological categories''
of the subject in stages: beginning with infons
and related structures
(Section~\ref{section-basic-infons}), and then later 
building situation structures 
(Section~\ref{section-truth-and-circumstance}).

In between these parts, we have a new
technical development a 
theory of {\it structural relations} introduced in 
Section~\ref{section-structural-relations}.   This
work is a generalisation of the notion of bisimulation
to more general structures. 
It also leads to treatments of partiality, parameters,
restriction,
and abstraction. 
 We believe that it simplifies
and unifies some existing ideas in the area.

\rem{
Second, we have new work on   logic on fact structures 
and situation structures in Section~\ref{section-truth-and-circumstance}.
This work
builds on traditional logical work on consequence relations,
and sequent calculi.
However, setting this work in the more relativized 
areas of situation theory is new.
}

\rem{
The main organizational point of this chapter 
comes from the idea of {\it controlled relativisation}.
Semantics faces the constant problem of relative vs.
absolute notions of truth. 
}

This chapter is intended to serve as an introduction 
to situation theory, not as a final say on the subject.
In writing it, we have tried to emphasize the overall
conceptual points of the subject, and to present a unified
theoretical account.   As a result,
we place in the background  many technical questions which
deserve much more attention.   



Situation Theory is the outcome of a history of reflection on
information structures in natural language studied by logical
means. The influential founding document Barwise and Perry (1983)
still bears a strong linguistic imprint, and their book led to
a research program called {\em Situation Semantics}. This Chapter
takes Situation Theory in an even broader sense to be the study of
information structure in general.  (Another manifestation
of this move is Devlin's book (Devlin 1991).) 
Nevertheless, examples and
motivations from natural language underlie much of what we
present here, and we shall also present some
pointers to the situation-semantic literature at the end of the chapter.



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