This chapter proposes a closer study of a very complicated question. It falls into two neat divisions, the discovery of materials and the use of them. Like most neat divisions this classification fails in practice; it fails for the simple reason that each process involves the other. The student cannot begin by making complete collection of his materials and then sit down to find out how to use them, for obvious reason that without this knowledge he cannot be sure that he has chosen the right materials to collect. The general nature and proper classification of material, the main classes and the sub-classes into which it may be divided, are matters very proper to be considered; and they may set about it either logically or practically. All material may be divided into two classes, primary sources and secondary sources. Information extracted from unpublished material carries with it curious charm, if it is acquired laboriously and from an obscure source.