High Quality Content by WIKIPEDIA articles! In library and information science documents (such as books, articles and pictures) are classified and searched by subject - as well as by other attributes such as author, genre and document type. This makes "subject" a fundamental term in this field. Library and information specialists assign subject labels to documents to make them findable. There are many ways to do this and in general there is not always consensus about which subject should be assigned to a given document. To optimize subject indexing and searching, we need to have a deeper understanding of what a subject is. The question: "what is to be understood by the statement `document A belongs to subject category X`?" has been debated in the field for more than 100 years (cf., below).