By the comparison of different societies which resemble or differ from each other in a given department (religion or government), with the object of discovering in what other departments they resemble or differ from each other, it is possible that interesting empirical results might be obtained. But, in order to _explain_ the _consensus_, it is necessary to work back to the facts which have produced it, the common causes of the various habits. We are thus obliged to undertake the investigation of causes, and we enter the province of what is called _philosophical_ history, because it investigates what was formerly called the _philosophy_ of facts--that is to say, their permanent relations. IX. The necessity of rising above the simple determination of facts in order to _explain_ them by their _causes_, a necessity which has governed the development of all the sciences, has at length been felt even in the study of history. Hence have arisen systematic philosophies of history, and attempts to discover historical laws and causes. We cannot here enter into a critical examination of these attempts, which the nineteenth century has produced in so great number; we shall merely indicate what are the ways in which the problem has been attacked, and what obstacles have prevented a scientific solution from being reached. The most natural method of explanation consists in the assumption that a transcendental cause, Providence, guides the whole course of events towards an end which is known to God.[209] This explanation can be but a metaphysical doctrine, crowning the work of science; for the distinguishing feature of science is that it only studies efficient causes. The historian is not called upon to investigate the first cause or final causes any more than the chemist or the naturalist. And, in fact, few writers on history nowadays stop to discuss the theory of Providence in its theological form. But the tendency to explain historical facts by transcendental causes survives in more modern theories in which metaphysic is disguised under scientific forms. The historians of the nineteenth century have been so strongly influenced by their philosophical education that most of them, sometimes unconsciously, introduce metaphysical formulæ into the construction of history. It will be enough to enumerate these systems, and point out their metaphysical character, so that reflecting historians may be warned to distrust them.