But perhaps no more expressive summary of the real relation between man and nature, not only during the Greek period but for many centuries after it, could be given than the fact that these three so-called elements which begin the Greek physical philosophy remained themselves unknown for more than two thousand years after Thales and Anaximenes and Heraclitus, until the very last century when, with the discovery of oxygen, men are able to prove that they are not elements at all; but that what we call fire is merely an effect of the rapid union of oxygen with bodies, while water and air are compounds of it with other gases. It is perfectly true that in the years between Thales and the death of Aristotle, a considerable body of physical facts had been accumulated; that Pythagoras had observed a number of acoustic phenomena and mathematically formulated their relations; it is true that--without detaining you to specify intermediate inquirers--we have that wonderful summary of Aristotle--wonderful for one man--which is contained in his _Physics_, from which the name "meta-physics" originated, though the circumstance that he placed the other books _after_ those on physics, calling them Τὰ μετὰ τὰ φυσιχὰ βιβλἱα, the meta-physical, or over and above physical, books. When we read the titles of these productions--here are "Eight Books of Physical Lectures," "Four Books of the Heavens," "Two Books of Production and Destruction," Treatises "On Animals," "On Plants," "On Colors," "On Sound"--we feel that we must be in a veritable realm of physical science. But if we examine these lectures and treatises, which probably contain the entire body of Greek physical learning, we find them hampered by a certain disability which seems to me characteristic not only of Greek thought, but of all man's early speculation, and which excludes the possibility of a fruitful and progressive physical science. I do not know how to characterize this disability otherwise than by calling it a lack of that sense of personal relation to fact which makes the thinker passionately and supremely solicitous about the truth, that is, the existence of his facts and the soundness of his logic; solicitous of these, not so much with reference to the value of his conclusions as because of an inward tender inexorable yearning for the truth and nothing but the truth. In short, I find that early thought everywhere, whether dealing with physical facts or meta-physical problems, is lacking in what I may call the intellectual conscience--the conscience, for example, which makes Mr. Darwin spend long and patient years in investigating small facts before daring to reason upon them; and which makes him state the facts adverse to his theory with as much care as the facts which make for it.