The simplest and easiest mode of classification is that which is founded on external conditions. Every historical fact belongs to a definite time and a definite place, and relates to a definite man or group of men: a convenient basis is thus afforded for the division and arrangement of facts. Classification of facts by their intrinsic nature was introduced very late, and has made way but slowly and imperfectly. It took its rise outside the domain of history, in certain branches of study dealing with special human phenomena, namely, language, literature, art, law, political economy, religion; studies which began by being dogmatic, but gradually assumed an historical character. Every human action is by its nature an individual transient phenomenon which is confined to a definite time and a definite place. In historical documents we find the contemporary names of groups, many of them resting on mere superficial resemblances.