Pericles, the Roman Empire, the Norman conquest of England, the French Revolution, or the administrations of George Washington--though, obviously, the character and amount of the contemporary materials of which one can avail himself varies enormously from people to people and from period to period. [Sidenote: Indirect character of all historical knowledge] History is unlike many other subjects of study in that our knowledge of it, at best, must come to us almost wholly through indirect means. That is to say, all our information regarding the past, and most of it regarding our own day, has to be obtained, in one form or another, through other people, or the remains that they have left behind them. No one of us can know much about even so recent an event as the Spanish-American War, except by reading newspapers, magazines and books, talking with men who had part in it, or listening to public addresses concerning it--all indirect means. And, of course, when we go back of the memory of men now living, say to the American Revolution, nobody can lay claim to an iota of knowledge which he has not acquired through indirect channels. In physics or chemistry, if a student desires, he can reproduce in the laboratory practically any phenomenon which he finds described in his books; he need not accept the mere word of his text or of his teacher, but can actually behold the thing with his own eyes. Such experimentation, however, has no place in the study of history, for by no sort of art can a Roman legion or a German comitatus or the battle of Hastings be reproduced before mortal eye. [Sidenote: An "historical source" defined] [Sidenote: Written sources]