Rule-based approaches
The basic idea behind rule-based approaches is that language obeys rules about how words are related to their meanings. For example, when we learn foreign languages, we typically learn specific rules about what words mean, how they’re ordered in sentences, and how prefixes and suffixes change the meanings of words. The rule-based approach to NLU operates on the premise that these kinds of rules can be provided to an NLU system so that the system can determine the meanings of sentences in the same way that a person does.
The rule-based approach was widely used in NLU from the mid-1950s through the mid-1990s until machine-learning-based approaches became popular. However, there are still NLU problems where rule-based approaches are useful, either on their own or when combined with other techniques.
We will begin by reviewing the rules and data that are relevant to various aspects of language.
Words and lexicons
Nearly everyone is familiar with...