A form of logic concerned with statements relative to the world and what it
  • Must be like
  • Might be like
  • Should be like
  • May be like
  • is known to be like
  • is believed to be like
  • was like or
  • will be like.
Therefore a rational agent is not limited to reasoning about one state of affairs.

A proposition is necessary if it holds in all possible worlds, and possible if it holds in some possible world. Truth may be defined in a possible world by constructing a model for a language that can describe nescessary and possible propositions.

One such is the Kripke Model.