agent? Nor was there any want, in those times, of well-authenticated stories of unearthly faces and forms seen in such solitudes. [Sidenote: Origin of psychology.] The modification made by Diogenes in the theory of Anaximenes, by converting it from a physical into a psychological system, is important, as marking the beginning of the special philosophy of Greece. The investigation of the intellectual development of the universe led the Greeks to the study of the intellect itself. In his special doctrine, Diogenes imputed the changeability of the air to its mobility; a property in which he thought it excelled all other substances, because it is among the rarest or thinnest of the elements. It is, however, said by some, who are disposed to transcendentalize his doctrine, that he did not mean the common atmospheric air, but something more attenuated and warm; and since, in its purest state, it constitutes the most perfect intellect, inferior degrees of reason must be owing to an increase of its density and moisture. Upon such a principle, the whole earth is animated by the breath of life; the souls of brutes, which differ from one another so much in intelligence, are only air in its various conditions of moisture and warmth. He explained the production of the world through condensation of the earth from air by cold, the warmth rising upward and forming the sun; in the stars he thought he recognized the respiratory organs of the world. From the preponderance of moist air in the constitution of brutes, he inferred that they are like the insane, incapable of thought, for thickness of the air impedes respiration, and therefore quick apprehension. From the fact that plants have no cavities wherein to receive the air, and are altogether unintelligent, he was led to the principle that the thinking power of man arises from the flowing of that substance throughout the body in the blood. He also explained the superior intelligence of men from their breathing a purer air than the beasts, which carry their nostrils near the ground. In these crude and puerile speculations we have the beginning of mental philosophy. [Sidenote: Modern discoveries as to the relations of the air.] [Sidenote: Inter-dependence of animals and plants.] [Sidenote: Agency of the sun.]