SEMANTICS

If syntax is concerned with the linguistic expressions of a sentence, then semantics is concerned with the relationship between the syntax of a sentence and reality. A semantically correct sentence reflects properties of the world, and as such makes it a hard task for a computer to check that every possible sentence is semantically correct.

A classic example of the difference between syntax and semantics is the sentence ``colourless green ideas sleep furiously'', which has correct syntactic structure, but semantically is meaningless. The adjective colourless implies that the noun phrase is without colour, but it combined with the adjective green, which implies that the noun phrase is green in colour. Clearly these two adjectives, although forming a syntactically correct structure, are semantically opposed, and along with other semantic violations present, make it an absurd sentence.

Contrast the above example with the sentence ``green sleep colourless furiously ideas'', which although has exactly the same words as the previous sentence, is syntactically incorrect making semantic analysis of the sentence useless.

Another way of illustrating the necessity for semantic processing is to think about several everyday linguistic tasks such as

• answering an exam question
• learning to use a piece of software from it's manual
• following a recipe

It should be clear that simply having syntactic information about the tasks listed above is not enough to accomplish any of them. The relationship between the linguistic elements involved to the non-linguistic knowledge of the world, is needed. For example in order to answer an essay question, background knowledge of the topic is a minimum requirement, whilst following a recipe requires knowledge about food and its preparation [13].

This section looks at requirements of any method that stores the relationship between phrases and their meaning.