Literature is an important part of academic papers, the writing material of academic papers and the basis of research. Literature is the carrier of academic inheritance and academic ethics. It is not only the embodiment of the author's rigorous academic spirit, but also an important basis for evaluating the academic level of the paper. Respecting documents means respecting predecessors' research, and respecting documents reflects the context of academic development.

The work of any paper is based on the work of others, so it is necessary to quote the work of others appropriately to meet academic norms. Literature can be divided into three levels according to the processing degree of knowledge, that is, whether its content, nature and structure have changed: first-class literature, second-class literature and third-class literature. Sometimes it also includes zero-time literature.

1. A document:

That is, the original literature usually refers to the author's originality. This kind of literature is specific, systematic, detailed, novel and creative, and it is the main object of literature retrieval. Monographs, periodical papers, dissertations, research reports, conference papers, patent specifications, etc. All belong to the same literature.

2. Secondary literature:

Commonly known as retrieval tools. It is formed by compressing a document and collecting, sorting and processing a scattered document. Because it provides clues and convenience for searching and using first-class literature, it is regarded as the main tool and means of literature retrieval. Title, table of contents, index, abstract, etc. They are all secondary documents.

3. Three documents:

Generally speaking, on the basis of using a large number of secondary documents, a large number of primary documents are selected, read and studied, and concentrated and refined. Three documents or comments on achievements, progress or predicted development trends, so that readers can understand the current research level and trends. Therefore, it is considered as an advanced intelligence. Yearbook, progress, review and summary all belong to three documents.

In the past thirty years, a new concept-zero-order literature has emerged. It is generally considered to be the information, knowledge and intelligence before the formation of the document, that is, the most primitive unpublished document. Such as letters, manuscripts, notes, records, even oral communication, practical methods and so on.

These informal documents are very valuable, and usually reflect the latest findings or problems encountered in the subject under study. It can help people to enlighten their thinking and generate new thinking, thus creating a higher level of scientific research. At present, scientists generally attach importance to the acquisition of zero-level documents.