What is the P source in Scripture?

A number of scholars believe that at least some books of the Old Testament have several authors (or sources). One of these sources is called "P." P is an individual or group of individuals who were associated with the priesthood. Thus, the title, "P." The affected biblical books were then supposedly put together by an editor, called a redactor, using the material written by P and other sources. Other scholars dispute the existence of P for at least two reasons: 1) There is no documentary evidence of any writing produced by P. 2) No two scholars seem to agree on exactly what parts of the Old Testament may have been written by P.