It would take me too far afield to give an account of the Greek schools which immediately succeeded the Ionic:  to tell of the Pythagoreans, who held that all things were constituted by numbers; of the Eleatics, who held that “only Being is,” and denied the possibility of change, thereby reducing the shifting panorama of the things about us to a mere delusive world of appearances; of Heraclitus, who was so impressed by the constant flux of things that he summed up his view of nature in the words:  “Everything flows”; of Empedocles, who found his explanation of the world in the combination of the four elements, since become traditional, earth, water, fire, and air; of Democritus, who developed a materialistic atomism which reminds one strongly of the doctrine of atoms as it has appeared in modern science; of Anaxagoras, who traced the system of things to the setting in order of an infinite multiplicity of different elements,—­“seeds of things,”—­which setting in order was due to the activity of the finest of things, Mind.

It is a delight to discover the illuminating thoughts which came to the minds of these men; and, on the other hand, it is amusing to see how recklessly they launched themselves on boundless seas when they were unprovided with chart and compass.  They were like brilliant children, who know little of the dangers of the great world, but are ready to undertake anything.  These philosophers regarded all knowledge as their province, and did not despair of governing so great a realm.  They were ready to explain the whole world and everything in it.  Of course, this can only mean that they had little conception of how much there is to explain, and of what is meant by scientific explanation.

It is characteristic of this series of philosophers that their attention was directed very largely upon the external world.  It was natural that this should be so.  Both in the history of the race and in that of the individual, we find that the attention is seized first by material things, and that it is long before a clear conception of the mind and of its knowledge is arrived at.  Observation precedes reflection.  When we come to think definitely about the mind, we are all apt to make use of notions which we have derived from our experience of external things.  The very words we use to denote mental operations are in many instances taken from this outer realm.  We “direct” the attention; we speak of “apprehension,” of “conception,” of “intuition.”  Our knowledge is “clear” or “obscure”; an oration is “brilliant”; an emotion is “sweet” or “bitter.”  What wonder that, as we read over the fragments that have come down to us from the Pre-Socratic philosophers, we should be struck by the fact that they sometimes leave out altogether and sometimes touch lightly upon a number of those things that we regard to-day as peculiarly within the province of the philosopher.  They busied themselves with the world as they saw it, and certain things had hardly as yet come definitely within their horizon.