1. A knowledge base system comprising: a storage unit in which a knowledge base is stored; and an operation unit configured to perform a logic operation on the knowledge base stored in said storage unit, wherein the knowledge base includes an entity identifier for identifying an entity and one or more attributes of the entity, the one or more attributes being associated with the entity identifier of the entity, the one or more attributes each include: an attribute identifier for identifying the attribute; and at least one of feature data and identification data, the feature data being one or more pieces of data representing the attribute and being associated with the attribute identifier for identifying the attribute, and the identification data being data associated with a word representing the attribute and being associated with the attribute identifier, the entity identifier is not a word representing the entity, and is made of a symbol that has no meaning per se, and the attribute identifier is not the word representing the attribute, and is made of a symbol that has no meaning per se.