The historiography of philosophy has already begun to open its arms, inviting and receiving the works of the historians. Every day it understands better that a history of Greek thought is not complete without taking count of Herodotus, Thucydides, and Polybius, nor of Roman thought without Livy and Tacitus, nor of the thought of the Renaissance without Machiavelli and Guicciardini. It must open them yet wider and clasp to its bosom even the humble medieval historiographers who noted the _Gesta episcoporum_ or _Historiolæ translationum_ or Vitæ sanctorum, or who bear witness to the Christian faith, according to their powers and in their own way, it is true, but not less than the great Augustine according to his powers. It must receive not only the hagiographical writers, but even obtuse philologists or sociologists who have amused us during the last decades and bear witness to the creed of positivism not other-wise than as Spencer or Haeckel in their systems. By means of this amplification of concepts and enrichment of material, the historiography of philosophy will place itself in the position of being able to show that philosophy is a force diffused throughout life, and not the particular invention and cult of certain men who are philosophers, and will obtain the means that have hitherto been lacking to effect a close conjunction with the whole historical movement. In its turn the history of historiography will gain by the fusion, because it will find its own directive principles in philosophy, and by its means will be rendered capable of understanding both the problems of history in general and those of its various aspects as history of art and of philosophy, of economic and moral life. To seek elsewhere the criterion of explanation is vain. Fueter, who takes a glance at the most recent historiography, that posterior to 1870, at the end of his book, discerns in it the new consciousness that gives the highest place to political and military power and marks the end of the old liberalism, the strengthening of such consciousness by means of the Darwinian theories concerning the struggle for life, the influence of a more intense economical and industrial life and a greater intensity of world politics, the repercussion of Egyptian and Orientalistic discoveries, which have aided in disproving the illusion of Europe as the centre of the world, the attraction exercised by the theory of races, and so on. These observations are just, but they do not reach the heart and brain of the most recent historiography; they merely revolve round its body. The heart or brain is, as I have observed, naturalism, the ideal of historical culture inspired and to be inspired by the natural sciences. So true is this that Fueter himself burns a few grains of incense before this idol, sighing for a form of history that shall be beautiful with the beauty of a well-made machine, rivalling a book on physics such as the _Theory of Tones_ of Helmholtz.