In linguistics and philosophy, modality refers to the ways language can express various relationships to reality or truth.
However, modal components have been identified in the meanings of countless natural language expressions, including counterfactuals, propositional attitudes, evidentials, habituals, and generics.
An expression like "obligatory" is said to have deontic flavor, since it discusses possibilities which are required given the laws or norms obeyed in reality.
The 'must' in this sentence thus expresses epistemic modality: "'for all we know', Agatha must be the murderer", where 'for all we know' is relative to some knowledge the speakers possess.
Research in these fields has led to a variety of accounts of the propositional content and conventional discourse effects of modal expressions.
Since the seminal work of Angelika Kratzer, formal semanticists have adopted a more finely grained notion of this set as determined by two conversational background functions called the modal base and ordering source respectively.
Assume for example that the speaker of sentence (4) above knows that John just bought a new luxury car and has rented a huge apartment.
In dynamic semantics, modals are analyzed as tests which check whether their prejacent is compatible with (or follows from) the information in the conversational common ground.
Probabilistic approaches motivated by gradable modal expressions provide a semantics which appeals to speaker credence in the prejacent.
As in most Standard European languages, the shape of the verb conveys not only information about modality, but also about other categories such as person and number of the subject.