This was originally posted on Saturday, September 14, 2013

The concept of a “situation” seems central to artificial intelligence and the semantics of natural language. The situation calculus (McCarthy and Hayes, 1969) underlies the notion of STRIPS planning used in the AI planning community.  The notion of “possible world” underlies the Kripke semantics of modal logic which is used in theories of the natural language semantics.  Situations (or possible worlds) play a central role in type-logical semantics.  “Situation Semantics” has been proposed as an alternative to possible world semantics where situations contain partial rather complete information. Situations seem closely related to the events of Davidsonian event semantics.  The notion of situation also seems closely related to the notion of state in reinforcement learning.

Before going any further, the first sentence of the lead story at at the moment of this writing is:

With the announcement on Saturday that the U.S. and Russia have reached an agreement on securing Syria’s chemical weapons stockpile, the American threat of U.S. military action was effectively taken off the table.

A search of with keywords “situation in Syria” finds the following title of a Sept 10 Washington Post article.

Syria situation further strains Obama’s relationship with the antiwar movement.

But what is “The Syria situation”.  Is the (actual) Syria situation a natural example of the general concept of a situation?  Should the first sentence above be considered to be a statement about, or true of, the Syria situation?  It seems to be true that the Syria situation evolves over time — there is the current Syria situation, a history of the Syria situation, and possible future outcomes of the Syria situation.

This may seem hopelessly complex.  But perhaps the complexity arises from a feeling that we need to define a system of concepts and rules that specify exactly what situations are.  Perhaps formal definitions and rules are not necessary.  Perhaps there are just soft (weighted) lexically-driven syntactic entailment rules where nothing is formally defined.

Quine took up the slogan “to be is to be the value of a variable“.  I would modify this and say “to be is to be the referent of a mention”.  If we think in terms of mention equivalence classes, as in mention-mention models of coreference, then the slogan becomes “to be is to be a mention equivalence class”. But in a database model of reality it seems more natural to work with a mention-entity model and use the term “reference” rather than coreference.  Of course Santa Clause and Unicorns do not exist in reality simply because we mention them.  However, they do exist in certain stories — in the situations described in certain works of fiction.  Does a fictional situation “exist” as a “possible world”?   I think the most intellectually coherent answer is yes — fictional worlds and the entities in them exist in much the same sense that the vector space R^100 exists, they are things we can think about.  I am not too concerned about contradictory entities such as a round square — the simple assumption that all mentions refer to some entity, where some entities may not be part of our physical reality, seems likely to work well for practical NLP systems.

Perhaps what really defines the notion of a situation is the fact that the same proposition — the same proposition entity or fluent — can have different truth values in different situations.  For example, the sentence “David is hungry” (for a fixed referent of “David”) is true in some situations but not in others. At a syntactic level propositions are interpreted relative to situations as in “When he arrived at the party last night, David was hungry”.  Here the phrase “when he arrived at the party last night” seems to be acting as a situation mention and “David was hungry” is being interpreted in the situation that is the referent of the situation mention.  I would extend the slogan “to be is to be the referent of a mention” with a second slogan “to be a situation is to be the referent of a situation mention”.  Somewhat more explicitly, a situation mention is a sentential modifier that intuitively gives the situation in which the sentence is to be interpreted.  For example, “Israel is only a minor player in the Syria situation”.

This is an informal blog post — there are no theorems here.  However, I hope that it provides food for thought.  My over-arching theme is that syntax plus reference (to database entities) plus, perhaps, lexically-driven syntactic paraphrase rules, covers a lot of “semantics”.

I promise that a significant fraction of future blog posts will discuss theorems.

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