The core of an expert system is the knowledge-base, a database in which the main domain-related intelligence is encoded. Such a database is typically populated using the knowledge of one or more human experts. In daylighting, such knowledge is in the form of heuristics, past experience, and ability to analyze data such as the results of a siulation or scale model study. However, heuristics and experience are only useful in very general situations, and they are not always reliable.