The realities of the past are things which we do not observe, and which we can only know in virtue of their resemblance to the realities of the present. In order to realise the conditions under which past events happened, we must observe the humanity of to-day, and look for the conditions under which analogous events happen now. History thus becomes an application of the descriptive sciences which deal with humanity, descriptive psychology, sociology or social science; but all these sciences are still but imperfectly established, and their defects retard the establishment of a science of history. Some of the conditions of human life are, however, so necessary and so obvious that the most superficial observation is enough to establish them. These are the conditions common to all humanity; they have their origin either in the physiological organisation which determines the material needs of men, or in the psychological organisation which determines their habits in matters of conduct. These conditions can therefore be provided for by the use of a set of general questions applicable to all the cases that may occur. It is with historical construction as with historical criticism--the impossibility of direct observation compels the use of prearranged sets of questions. The human actions which form the subject-matter of history differ from age to age and from country to country, just as men and societies have differed from each other; and, indeed, it is the special aim of history to study these differences. If men had always had the same form of government or spoken the same language, there would be no occasion to write the history of forms of government or the history of languages. But these differences are comprised within limits imposed by the general conditions of human life; they are but varieties of certain modes of being and doing which are common to the whole of humanity, or at least to the great majority of men. We cannot know _a priori_ what was the mode of government or the language of an historical people; it is the business of history to tell us. But that a given people had a language and had a form of government is something which we are entitled to assume, before examination, in every possible case. By drawing up the list of the fundamental phenomena which we may expect to find in the life of every individual and every people, we shall have suggested to us a set of general questions which will be summary, but still sufficient to enable us to arrange the bulk of historical facts in a certain number of natural groups, each of which will form a special branch of history. This scheme of general classification will supply the scaffolding of historical construction.