1. A system for sorting documents comprising: means responsive to a predetermined property of said documents for categorizing said documents into groups according to predetermined values of said property, said categorizing means including means responsive to the value of said property of each document for determining whether said value is unique to a particular class of documents, said uniqueness determining means including means for identifying said document when said value is unique to a particular class of documents; and means responsive to a predetermined characteristic of said documents other than said property for categorizing the documents having values of said property that are not unique to a particular class of documents into groups according to predetermined parameters of said characteristic and the value of said property, said characteristic responsive categorizing means including means responsive to the parameters of said characteristic and the value of said property of each document for determining whether the combination of said parameter of said value is unique to a particular class of documents and for identifying said document when said combination is unique to a particular class of documents.