1. When we speak of the Ionic Philosophy of Nature we do not mean to imply that the representatives of this Philosophy form what is called a "Philosophic School" in the strict sense of this term. There was no centre among them from which a common movement of thought spread abroad. We have to do only with a number of Philosophers who had a common subject of investigation -- Nature, and whose philosophic views had certain common characters. These Philosophers do not even belong without exception to the Ionian race. They do not form a sect acknowledging one founder whose doctrines they uphold; and therefore it is only by a somewhat strained use of the term that we can speak of an "Ionic School."
2. We can, however, divide these Philosophers of Nature into two classes -- the earlier and the later. The earlier (Ionic "Physiologists," phusiologoi) are the representatives of the Greek Philosophy of Nature in its rudimentary stage; while the later, having before them the works as well of the earlier Ionic Philosophers as of the Pythagoreans and Eleatics, were enabled to give this Philosophy a wider development. It is, however, worth noticing that the earlier Ionic Philosophers for the most part adopted a dynamical principle to explain the origin of things, while the later as generally incline to mechanical conceptions.
3. We shall treat, in order, first the earlier, and then the later Ionic Philosophers.
The earlier Ionic Philosophers had this in common, that in their inquiry as to how things in nature come into being and cease to be, they identified the active and the passive principles, the causa efficiens and the causa materialis, kai stocheion), and strove to explain the rise of the order of nature by a dynamical process from this principle. Their doctrines are thus fundamentally forms of Hylozoism (Doctrine of Animated Matter). Amongst the earlier Ionic Philosophers are to be numbered Thales of Miletus, Anaximander, Anaximenes, and Diogenes of Apollonia, whose theories bear chiefly on the primal material basis of all things; and Heraclitus who concerned himself mainly with the processes of origin and decay.
1. Thales of Miletus, of Phoenician extraction, born B.C. 640, is described by Aristotle (Met 1. 3.) as the founder of the Ionic Philosophy, and so the founder of Greek Philosophy as a whole. He is said to have studied Geometry in Greece; at least Proclus makes this statement regarding him (on Euclid, p. 19). He is furthermore credited with having foretold an eclipse of the sun which occurred during the reign of the Lydian King, Alyattes.
2. The fundamental theory of his Philosophy of Nature may be thus stated: -- Out of water all things are made. Water is the primal matter, and with this primal matter, the force which is active in nature is identified. From this primal matter, probably by a process of rarefaction or condensation, he derives the origin of all things. According to Aristotle (Met. 1. 3.) "Thales was perhaps led to this opinion by observing that the nutriment of all things is moist, that heat itself, by which living things are maintained in life, is educed from moisture, -- but that from which another thing is derived is a principle of that other thing -- and further by observing that the seed (from which living organisms spring) is of its nature moist. But the principle making moist objects moist is water." In consequence of this view Thales could regard all things as penetrated and vivified by the Divine power, and in this sense could say that the gods filled all things panta plêrê Theon einai. (Arist. de anim. 1. 5.) He held the magnet to be animated because of its attraction of iron. He was of opinion that the earth floated upon water.
3. In later times Hippo of Samos or of Rhegium -- a Physicist of the time of Pericles, who seems to have lived for a considerable time at Athens, adopted the theory of Thales. He discovers in water, or the moist element, the ultimate principle of all things. He does not seem to have attracted much attention. Aristotle mentions him but seldom, and not always in terms of praise. (De anim. 1. 2. Met. 1. 3.)
1. Anaximander of Miletus (born about B.C. 611), was the first of the Greeks to compose a treatise "On Nature." The primal basis of all being (arche), and out of which all things came forth is, in his view, the Unlimited (to apeiron). From this apeiron all things derive their origin. At first it differentiates itself into the opposing elements, hot and cold, moist and dry -- kindred elements standing in antithesis. "As a result of a perpetual movement of revolution, condensations of the air are effected, and in this way numberless worlds come into being -- heavenly divinities -- in the midst of which the earth, cylindrical in form, maintains itself at rest owing to its being equally distant from all points of the heavenly sphere." The earth was evolved from the primeval moisture under the influence of heat emanating from the sun, and, fecundated by heat, it gave birth to living beings. The latter thus derive their being from the element of moisture, and this explains why the creatures now living on the land were originally of the fish kind, and acquired their present form only as the surface of the earth became dry. It is said that Anaximander described the soul as of gaseous nature. All things come forth from the apeiron and all things are fated to return to it again.
2. With regard to the question, what Anaximander really meant by the apeiron, opinions are divided. Some (Ritter) maintain that he understood by the term a congeries of the primary elements; that the origin of things from the apeiron is nothing more than a separation of elements, and that thus the evolution of the order of nature is, in his theory, a purely mechanical process. Others (Herbart) are of opinion that Anaximander meant by the apeiron a primary matter indeterminate in quality and unlimited in quantity, and that he thus conceived the evolution of the natural order to be a dynamical process. Aristotle, it must be admitted, speaks of a migma Anaximandrou (Met. 12. 2.), but he also mentions (Phys. 3. 4.) that Anaximander taught that the apeiron was divine, embracing all and controlling all -- a notion which best accords with a dynamical theory. The latter was more probably the theory of this Philosopher. It would, however, appear that Anaximander was not very explicit in his teaching as to the nature of the apeiron, and that Aristotle was thus unable to set forth his doctrines with assured accuracy.
3. Anaximenes of Miletus, a successor of Anaximander, perhaps his pupil (about B.C. 528), held air to be the primary principle of all things. "As the soul within us," he says, "which is air, holds our being together, so does the breath and the air embrace the world." -- (Stob. Eclog. Phys. p. 296). This air, infinite in extension, is instinct with life, i.e., it is not merely the material, it is also the efficient cause of all things. Out of this primary being, by the process of condensation (puknôsis) and rarefaction (manôsis or araiôsis) are derived all other things -- fire, wind, clouds, water, earth. The earth -- a smooth mass of circular outline, and the earliest of the formations of the Universe -- is supported by the air. Anaximander describes this infinite primal principle of things as the Deity, though he also speaks of other gods who have derived their being from it.
4. This view of Anaximenes, with regard to the first principle of all things, was also held by Diogenes of Apollonia, a philosopher who lived in the fifth century before Christ. He holds the air to be the primary principle and permanent basis of all things. He discovers a proof that all substance is one in the fact of the assimilation by plants of the various elements of the earth's crust, and of the elements of the vegetable world by animal organisms (Simpl. in Phys. fol. 32 B). The same theories were held by another philosopher, Idaeus of Himera, of whom nothing further is known.