The Principle of Compositionality
The analysis of phrase and sentence meaning is based on the Principle of Compositionality that is usually ascribed to Gottlob Frege (Frege's principle of meaning, Fregean principle). According to this principle, it is assumed that the whole meaning of a phrase or sentence can be described according to the functional interdependency of the meaning of its parts. Thus, the meaning of a phrase or sentence can be reconstructed by the meaning of its component parts and their syntactic relationship to one another.
Source of the cartoon: cse.buffalo.edu
In general, the Principle of Compositionality is based on the assumption that the interpretation of sentence meaning includes aspects of lexical semantics as well as syntax. Thus, the same words can be combined differently to form sentences with a different meaning, as in the sentences The dog chased the cat and The cat chased the dog. In many cases, even identical sequences of words can have more than one possible meaning and are thus referred to as structurally ambiguous. Such a structural ambiguity is found in sentences with more than one possible reading, as in sentences like The reporter talked about an explosion in Paris.
(For a syntactic analysis of this sentence, consult the page on ambiguous VPs in the syntax module!)
Besides our knowledge of word meanings and syntactic structures, there are even more factors involved in the interpretation of sentence meaning. In many cases, implicit assumptions called presuppositions play an important role. Presuppositions, i.e. certain attitudes and beliefs of the speaker and the addressee, are different from entailment since they also hold true when the presupposing sentence is negated: The sentences John managed to pass the exam and John didn't manage to pass the exam both presuppose John tried to pass the exam. There are a number of presupposition triggers that are regularly associated with certain assumptions (e.g. manage, realize, regret). As presuppositions play an important role in the interpretation of contextual meaning, they are said to be on the semantics - pragmatics interface.