The exact position which Anaxagoras had among his contemporaries, and his exact place in the development of philosophy, have always been somewhat in dispute. It is not known, of a certainty, that he even held an open school at Athens. Ritter thinks it doubtful that he did. It was his fate to be misunderstood, or underestimated, by Aristotle; that in itself would have sufficed greatly to dim his fame - might, indeed, have led to his almost entire neglect had he not been a truly remarkable thinker. With most of the questions that have exercised the commentators we have but scant concern. Following Aristotle, most historians of philosophy have been metaphysicians; they have concerned themselves far less with what the ancient thinkers really knew than with what they thought. A chance using of a verbal quibble, an esoteric phrase, the expression of a vague mysticism - these would suffice to call forth reams of exposition. It has been the favorite pastime of historians to weave their own anachronistic theories upon the scanty woof of the half- remembered thoughts of the ancient philosophers. To make such cloth of the imagination as this is an alluring pastime, but one that must not divert us here. Our point of view reverses that of the philosophers. We are chiefly concerned, not with some vague saying of Anaxagoras, but with what he really knew regarding the phenomena of nature; with what he observed, and with the comprehensible deductions that he derived from his observations. In attempting to answer these inquiries, we are obliged, in part, to take our evidence at second-hand; but, fortunately, some fragments of writings of Anaxagoras have come down to us. We are told that he wrote only a single book. It was said even (by Diogenes) that he was the first man that ever wrote a work in prose. The latter statement would not bear too close an examination, yet it is true that no extensive prose compositions of an earlier day than this have been preserved, though numerous others are known by their fragments. Herodotus, "the father of prose," was a slightly younger contemporary of the Clazomenaean philosopher; not unlikely the two men may have met at Athens.

Notwithstanding the loss of the greater part of the writings of Anaxagoras, however, a tolerably precise account of his scientific doctrines is accessible. Diogenes Laertius expresses some of them in very clear and precise terms. We have already pointed out the uncertainty that attaches to such evidence as this, but it is as valid for Anaxagoras as for another. If we reject such evidence, we shall often have almost nothing left; in accepting it we may at least feel certain that we are viewing the thinker as his contemporaries and immediate successors viewed him. Following Diogenes, then, we shall find some remarkable scientific opinions ascribed to Anaxagoras. "He asserted," we are told, "that the sun was a mass of burning iron, greater than Peloponnesus, and that the moon contained houses and also hills and ravines." In corroboration of this, Plato represents him as having conjectured the right explanation of the moon's light, and of the solar and lunar eclipses. He had other astronomical theories that were more fanciful; thus "he said that the stars originally moved about in irregular confusion, so that at first the pole-star, which is continually visible, always appeared in the zenith, but that afterwards it acquired a certain declination, and that the Milky Way was a reflection of the light of the sun when the stars did not appear. The comets he considered to be a concourse of planets emitting rays, and the shooting- stars he thought were sparks, as it were, leaping from the firmament."

Much of this is far enough from the truth, as we now know it, yet all of it shows an earnest endeavor to explain the observed phenomena of the heavens on rational principles. To have predicated the sun as a great molten mass of iron was indeed a wonderful anticipation of the results of the modern spectroscope. Nor can it be said that this hypothesis of Anaxagoras was a purely visionary guess. It was in all probability a scientific deduction from the observed character of meteoric stones. Reference has already been made to the alleged prediction of the fall of the famous meteor at aegespotomi by Anaxagoras. The assertion that he actually predicted this fall in any proper sense of the word would be obviously absurd. Yet the fact that his name is associated with it suggests that he had studied similar meteorites, or else that he studied this particular one, since it is not quite clear whether it was before or after this fall that he made the famous assertion that space is full of falling stones. We should stretch the probabilities were we to assert that Anaxagoras knew that shooting-stars and meteors were the same, yet there is an interesting suggestiveness in his likening the shooting-stars to sparks leaping from the firmament, taken in connection with his observation on meteorites. Be this as it may, the fact that something which falls from heaven as a blazing light turns out to be an iron-like mass may very well have suggested to the most rational of thinkers that the great blazing light called the sun has the same composition. This idea grasped, it was a not unnatural extension to conceive the other heavenly bodies as having the same composition.

This led to a truly startling thought. Since the heavenly bodies are of the same composition as the earth, and since they are observed to be whirling about the earth in space, may we not suppose that they were once a part of the earth itself, and that they have been thrown off by the force of a whirling motion? Such was the conclusion which Anaxagoras reached; such his explanation of the origin of the heavenly bodies. It was a marvellous guess. Deduct from it all that recent science has shown to be untrue; bear in mind that the stars are suns, compared with which the earth is a mere speck of dust; recall that the sun is parent, not daughter, of the earth, and despite all these deductions, the cosmogonic guess of Anaxagoras remains, as it seems to us, one of the most marvellous feats of human intelligence. It was the first explanation of the cosmic bodies that could be called, in any sense, an anticipation of what the science of our own day accepts as a true explanation of cosmic origins. Moreover, let us urge again that this was no mere accidental flight of the imagination; it was a scientific induction based on the only data available; perhaps it is not too much to say that it was the only scientific induction which these data would fairly sustain. Of course it is not for a moment to be inferred that Anaxagoras understood, in the modern sense, the character of that whirling force which we call centrifugal. About two thousand years were yet to elapse before that force was explained as elementary inertia; and even that explanation, let us not forget, merely sufficed to push back the barriers of mystery by one other stage; for even in our day inertia is a statement of fact rather than an explanation.

But however little Anaxagoras could explain the centrifugal force on mechanical principles, the practical powers of that force were sufficiently open to his observation. The mere experiment of throwing a stone from a sling would, to an observing mind, be full of suggestiveness. It would be obvious that by whirling the sling about, the stone which it held would be sustained in its circling path about the hand in seeming defiance of the earth's pull, and after the stone had left the sling, it could fly away from the earth to a distance which the most casual observation would prove to be proportionate to the speed of its flight. Extremely rapid motion, then, might project bodies from the earth's surface off into space; a sufficiently rapid whirl would keep them there. Anaxagoras conceived that this was precisely what had occurred. His imagination even carried him a step farther - to a conception of a slackening of speed, through which the heavenly bodies would lose their centrifugal force, and, responding to the perpetual pull of gravitation, would fall back to the earth, just as the great stone at aegespotomi had been observed to do.

Here we would seem to have a clear conception of the idea of universal gravitation, and Anaxagoras stands before us as the anticipator of Newton. Were it not for one scientific maxim, we might exalt the old Greek above the greatest of modern natural philosophers; but that maxim bids us pause. It is phrased thus, "He discovers who proves." Anaxagoras could not prove; his argument was at best suggestive, not demonstrative. He did not even know the laws which govern falling bodies; much less could he apply such laws, even had he known them, to sidereal bodies at whose size and distance he could only guess in the vaguest terms. Still his cosmogonic speculation remains as perhaps the most remarkable one of antiquity. How widely his speculation found currency among his immediate successors is instanced in a passage from Plato, where Socrates is represented as scornfully answering a calumniator in these terms: "He asserts that I say the sun is a stone and the moon an earth. Do you think of accusing Anaxagoras, Miletas, and have you so low an opinion of these men, and think them so unskilled in laws, as not to know that the books of Anaxagoras the Clazomenaean are full of these doctrines. And forsooth the young men are learning these matters from me which sometimes they can buy from the orchestra for a drachma, at the most, and laugh at Socrates if he pretends they are his-particularly seeing they are so strange."

The element of error contained in these cosmogonic speculations of Anaxagoras has led critics to do them something less than justice. But there is one other astronomical speculation for which the Clazomenaean philosopher has received full credit. It is generally admitted that it was he who first found out the explanation of the phases of the moon; a knowledge that that body shines only by reflected light, and that its visible forms, waxing and waning month by month from crescent to disk and from disk to crescent, merely represent our shifting view of its sun-illumined face. It is difficult to put ourselves in the place of the ancient observer and realize how little the appearances suggest the actual fact. That a body of the same structure as the earth should shine with the radiance of the moon merely because sunlight is reflected from it, is in itself a supposition seemingly contradicted by ordinary experience. It required the mind of a philosopher, sustained, perhaps, by some experimental observations, to conceive the idea that what seems so obviously bright may be in reality dark. The germ of the conception of what the philosopher speaks of as the noumena, or actualities, back of phenomena or appearances, had perhaps this crude beginning. Anaxagoras could surely point to the moon in support of his seeming paradox that snow, being really composed of water, which is dark, is in reality black and not white - a contention to which we shall refer more at length in a moment.

But there is yet another striking thought connected with this new explanation of the phases of the moon. The explanation implies not merely the reflection of light by a dark body, but by a dark body of a particular form. Granted that reflections are in question, no body but a spherical one could give an appearance which the moon presents. The moon, then, is not merely a mass of earth, it is a spherical mass of earth. Here there were no flaws in the reasoning of Anaxagoras. By scientific induction he passed from observation to explanation. A new and most important element was added to the science of astronomy.

Looking back from the latter-day stand-point, it would seem as if the mind of the philosopher must have taken one other step: the mind that had conceived sun, moon, stars, and earth to be of one substance might naturally, we should think, have reached out to the further induction that, since the moon is a sphere, the other cosmic bodies, including the earth, must be spheres also. But generalizer as he was, Anaxagoras was too rigidly scientific a thinker to make this assumption. The data at his command did not, as he analyzed them, seem to point to this conclusion. We have seen that Pythagoras probably, and Parmenides surely, out there in Italy had conceived the idea of the earth's rotundity, but the Pythagorean doctrines were not rapidly taken up in the mother- country, and Parmenides, it must be recalled, was a strict contemporary of Anaxagoras himself. It is no reproach, therefore, to the Clazomenaean philosopher that he should have held to the old idea that the earth is flat, or at most a convex disk - the latter being the Babylonian conception which probably dominated that Milesian school to which Anaxagoras harked back.

Anaxagoras may never have seen an eclipse of the moon, and even if he had he might have reflected that, from certain directions, a disk may throw precisely the same shadow as a sphere. Moreover, in reference to the shadow cast by the earth, there was, so Anaxagoras believed, an observation open to him nightly which, we may well suppose, was not without influence in suggesting to his mind the probable shape of the earth. The Milky Way, which doubtless had puzzled astronomers from the beginnings of history and which was to continue to puzzle them for many centuries after the day of Anaxagoras, was explained by the Clazomenaean philosopher on a theory obviously suggested by the theory of the moon's phases. Since the earth- like moon shines by reflected light at night, and since the stars seem obviously brighter on dark nights, Anaxagoras was but following up a perfectly logical induction when he propounded the theory that the stars in the Milky Way seem more numerous and brighter than those of any other part of the heavens, merely because the Milky Way marks the shadow of the earth. Of course the inference was wrong, so far as the shadow of the earth is concerned; yet it contained a part truth, the force of which was never fully recognized until the time of Galileo. This consists in the assertion that the brightness of the Milky Way is merely due to the glow of many stars. The shadow- theory of Anaxagoras would naturally cease to have validity so soon as the sphericity of the earth was proved, and with it, seemingly, fell for the time the companion theory that the Milky Way is made up of a multitude of stars.

It has been said by a modern critic[1] that the shadow-theory was childish in that it failed to note that the Milky Way does not follow the course of the ecliptic. But this criticism only holds good so long as we reflect on the true character of the earth as a symmetrical body poised in space. It is quite possible to conceive a body occupying the position of the earth with reference to the sun which would cast a shadow having such a tenuous form as the Milky Way presents. Such a body obviously would not be a globe, but a long-drawn-out, attenuated figure. There is, to be sure, no direct evidence preserved to show that Anaxagoras conceived the world to present such a figure as this, but what we know of that philosopher's close-reasoning, logical mind gives some warrant to the assumption - gratuitous though in a sense it be - that the author of the theory of the moon's phases had not failed to ask himself what must be the form of that terrestrial body which could cast the tenuous shadow of the Milky Way. Moreover, we must recall that the habitable earth, as known to the Greeks of that day, was a relatively narrow band of territory, stretching far to the east and to the west.

 Anaxagoras as Meteorologist.
The man who had studied the meteorite of aegospotami, and been put by it on the track of such remarkable inductions, was, naturally, not oblivious to the other phenomena of the atmosphere. Indeed, such a mind as that of Anaxagoras was sure to investigate all manner of natural phenomena, and almost equally sure to throw new light on any subject that it investigated. Hence it is not surprising to find Anaxagoras credited with explaining the winds as due to the rarefactions of the atmosphere produced by the sun. This explanation gives Anaxagoras full right to be called "the father of meteorology," a title which, it may be, no one has thought of applying to him, chiefly because the science of meteorology did not make its real beginnings until some twenty-four hundred years after the death of its first great votary. Not content with explaining the winds, this prototype of Franklin turned his attention even to the tipper atmosphere. "Thunder," he is reputed to have said, "was produced by the collision of the clouds, and lightning by the rubbing together of the clouds." We dare not go so far as to suggest that this implies an association in the mind of Anaxagoras between the friction of the clouds and the observed electrical effects generated by the friction of such a substance as amber. To make such a suggestion doubtless would be to fall victim to the old familiar propensity to read into Homer things that Homer never knew. Yet the significant fact remains that Anaxagoras ascribed to thunder and to lightning their true position as strictly natural phenomena. For him it was no god that menaced humanity with thundering voice and the flash of his divine fires from the clouds. Little wonder that the thinker whose science carried him to such scepticism as this should have felt the wrath of the superstitious Athenians.

 Biological Speculations.
Passing from the phenomena of the air to those of the earth itself, we learn that Anaxagoras explained an earthquake as being produced by the returning of air into the earth. We cannot be sure as to the exact meaning here, though the idea that gases are imprisoned in the substance of the earth seems not far afield. But a far more remarkable insight than this would imply was shown by Anaxagoras when he asserted that a certain amount of air is contained in water, and that fishes breathe this air. The passage of Aristotle in which this opinion is ascribed to Anaxagoras is of sufficient interest to be quoted at length:

"Democritus, of Abdera," says Aristotle, "and some others, that have spoken concerning respiration, have determined nothing concerning other animals, but seem to have supposed that all animals respire. But Anaxagoras and Diogenes (Apolloniates), who say that all animals respire, have also endeavored to explain how fishes, and all those animals that have a hard, rough shell, such as oysters, mussels, etc., respire. And Anaxagoras, indeed, says that fishes, when they emit water through their gills, attract air from the mouth to the vacuum in the viscera from the water which surrounds the mouth; as if air was inherent in the water."[2]

It should be recalled that of the three philosophers thus mentioned as contending that all animals respire, Anaxagoras was the elder; he, therefore, was presumably the originator of the idea. It will be observed, too, that Anaxagoras alone is held responsible for the idea that fishes respire air through their gills, "attracting" it from the water. This certainly was one of the shrewdest physiological guesses of any age, if it be regarded as a mere guess. With greater justice we might refer to it as a profound deduction from the principle of the uniformity of nature.

In making such a deduction, Anaxagoras was far in advance of his time as illustrated by the fact that Aristotle makes the citation we have just quoted merely to add that "such things are impossible," and to refute these "impossible" ideas by means of metaphysical reasonings that seemed demonstrative not merely to himself, but to many generations of his followers.

We are told that Anaxagoras alleged that all animals were originally generated out of moisture, heat, and earth particles. Just what opinion he held concerning man's development we are not informed. Yet there is one of his phrases which suggests - without, perhaps, quite proving - that he was an evolutionist. This phrase asserts, with insight that is fairly startling, that man is the most intelligent of animals because he has hands. The man who could make that assertion must, it would seem, have had in mind the idea of the development of intelligence through the use of hands - an idea the full force of which was not evident to subsequent generations of thinkers until the time of Darwin.

 Physical Speculations.
Anaxagoras is cited by Aristotle as believing that "plants are animals and feel pleasure and pain, inferring this because they shed their leaves and let them grow again." The idea is fanciful, yet it suggests again a truly philosophical conception of the unity of nature. The man who could conceive that idea was but little hampered by traditional conceptions. He was exercising a rare combination of the rigidly scientific spirit with the poetical imagination. He who possesses these gifts is sure not to stop in his questionings of nature until he has found some thinkable explanation of the character of matter itself. Anaxagoras found such an explanation, and, as good luck would have it, that explanation has been preserved. Let us examine his reasoning in some detail. We have already referred to the claim alleged to have been made by Anaxagoras that snow is not really white, but black. The philosopher explained his paradox, we are told, by asserting that snow is really water, and that water is dark, when viewed under proper conditions - as at the bottom of a well. That idea contains the germ of the Clazomenaean philosopher's conception of the nature of matter. Indeed, it is not unlikely that this theory of matter grew out of his observation of the changing forms of water. He seems clearly to have grasped the idea that snow on the one hand, and vapor on the other, are of the same intimate substance as the water from which they are derived and into which they may be again transformed. The fact that steam and snow can be changed back into water, and by simple manipulation cannot be changed into any other substance, finds, as we now believe, its true explanation in the fact that the molecular structure, as we phrase it - that is to say, the ultimate particle of which water is composed, is not changed, and this is precisely the explanation which Anaxagoras gave of the same phenomena. For him the unit particle of water constituted an elementary body, uncreated, unchangeable, indestructible. This particle, in association with like particles, constitutes the substance which we call water. The same particle in association with particles unlike itself, might produce totally different substances - as, for example, when water is taken up by the roots of a plant and becomes, seemingly, a part of the substance of the plant. But whatever the changed association, so Anaxagoras reasoned, the ultimate particle of water remains a particle of water still. And what was true of water was true also, so he conceived, of every other substance. Gold, silver, iron, earth, and the various vegetables and animal tissues - in short, each and every one of all the different substances with which experience makes us familiar, is made up of unit particles which maintain their integrity in whatever combination they may be associated. This implies, obviously, a multitude of primordial particles, each one having an individuality of its own; each one, like the particle of water already cited, uncreated, unchangeable, and indestructible.

Fortunately, we have the philosopher's own words to guide us as to his speculations here. The fragments of his writings that have come down to us (chiefly through the quotations of Simplicius) deal almost exclusively with these ultimate conceptions of his imagination. In ascribing to him, then, this conception of diverse, uncreated, primordial elements, which can never be changed, but can only be mixed together to form substances of the material world, we are not reading back post-Daltonian knowledge into the system of Anaxagoras. Here are his words: "The Greeks do not rightly use the terms 'coming into being' and 'perishing.' For nothing comes into being, nor, yet, does anything perish; but there is mixture and separation of things that are. So they would do right in calling 'coming into being' 'mixture' and 'perishing' 'separation.' For how could hair come from what is not hair? Or flesh from what is not flesh?"

Elsewhere he tells us that (at one stage of the world's development) "the dense, the moist, the cold, the dark, collected there where now is earth; the rare, the warm, the dry, the bright, departed towards the further part of the aether. The earth is condensed out of these things that are separated, for water is separated from the clouds, and earth from the water; and from the earth stones are condensed by the cold, and these are separated farther from the water." Here again the influence of heat and cold in determining physical qualities is kept pre-eminently in mind. The dense, the moist, the cold, the dark are contrasted with the rare, the warm, the dry, and bright; and the formation of stones is spoken of as a specific condensation due to the influence of cold. Here, then, we have nearly all the elements of the Daltonian theory of atoms on the one hand, and the nebular hypothesis of Laplace on the other. But this is not quite all. In addition to such diverse elementary particles as those of gold, water, and the rest, Anaxagoras conceived a species of particles differing from all the others, not merely as they differ from one another, but constituting a class by themselves; particles infinitely smaller than the others; particles that are described as infinite, self-powerful, mixed with nothing, but existing alone. That is to say (interpreting the theory in the only way that seems plausible), these most minute particles do not mix with the other primordial particles to form material substances in the same way in which these mixed with one another. But, on the other hand, these "infinite, self-powerful, and unmixed" particles commingle everywhere and in every substance whatever with the mixed particles that go to make up the substances.

There is a distinction here, it will be observed, which at once suggests the modern distinction between physical processes and chemical processes, or, putting it otherwise, between molecular processes and atomic processes; but the reader must be guarded against supposing that Anaxagoras had any such thought as this in mind. His ultimate mixable particles can be compared only with the Daltonian atom, not with the molecule of the modern physicist, and his "infinite, self- powerful, and unmixable" particles are not comparable with anything but the ether of the modern physicist, with which hypothetical substance they have many points of resemblance. But the "infinite, self- powerful, and unmixed" particles constituting thus an ether-like plenum which permeates all material structures, have also, in the mind of Anaxagoras, a function which carries them perhaps a stage beyond the province of the modern ether. For these "infinite, self powerful, and unmixed" particles are imbued with, and, indeed, themselves constitute, what Anaxagoras terms nous, a word which the modern translator has usually paraphrased as "mind." Neither that word nor any other available one probably conveys an accurate idea of what Anaxagoras meant to imply by the word nous. For him the word meant not merely "mind" in the sense of receptive and comprehending intelligence, but directive and creative intelligence as well. Again let Anaxagoras speak for himself: "Other things include a portion of everything, but nous is infinite, and self-powerful, and mixed with nothing, but it exists alone, itself by itself. For if it were not by itself, but were mixed with anything else, it would include parts of all things, if it were mixed with anything; for a portion of everything exists in every thing, as has been said by me before, and things mingled with it would prevent it from having power over anything in the same way that it does now that it is alone by itself. For it is the most rarefied of all things and the purest, and it has all knowledge in regard to everything and the greatest power; over all that has life, both greater and less, nous rules. And nous ruled the rotation of the whole, so that it set it in rotation in the beginning. First it began the rotation from a small beginning, then more and more was included in the motion, and yet more will be included. Both the mixed and the separated and distinct, all things nous recognized. And whatever things were to be, and whatever things were, as many as are now, and whatever things shall be, all these nous arranged in order; and it arranged that rotation, according to which now rotate stars and sun and moon and air and aether, now that they are separated. Rotation itself caused the separation, and the dense is separated from the rare, the warm from the cold, the bright from the dark, the dry from the moist. And when nous began to set things in motion, there was separation from everything that was in motion, all this was made distinct. The rotation of the things that were moved and made distinct caused them to be yet more distinct."[3]

Nous, then, as Anaxagoras conceives it, is "the most rarefied of all things, and the purest, and it has knowledge in regard to everything and the greatest power; over all that has life, both greater and less, it rules." But these are postulants of omnipresence and omniscience. In other words, nous is nothing less than the omnipotent artificer of the material universe. It lacks nothing of the power of deity, save only that we are not assured that it created the primordial particles. The creation of these particles was a conception that for Anaxagoras, as for the modern Spencer, lay beyond the range of imagination. Nous is the artificer, working with "uncreated" particles. Back of nous and the particles lies, for an Anaxagoras as for a Spencer, the Unknowable. But nous itself is the equivalent of that universal energy of motion which science recognizes as operating between the particles of matter, and which the theologist personifies as Deity. It is Pantheistic deity as Anaxagoras conceives it; his may be called the first scientific conception of a non- anthropomorphic god. In elaborating this conception Anaxagoras proved himself one of the most remarkable scientific dreamers of antiquity. To have substituted for the Greek Pantheon of anthropomorphic deities the conception of a non-anthropomorphic immaterial and ethereal entity, of all things in the world "the most rarefied and the purest," is to have performed a feat which, considering the age and the environment in which it was accomplished, staggers the imagination. As a strictly scientific accomplishment the great thinker's conception of primordial elements contained a germ of the truth which was to lie dormant for 2200 years, but which then, as modified and vitalized by the genius of Dalton, was to dominate the new chemical science of the nineteenth century. If there are intimations that the primordial element of Anaxagoras and of Dalton may turn out in the near future to be itself a compound, there will still remain the yet finer particles of the nous of Anaxagoras to baffle the most subtle analysis of which to-day's science gives us any pre-vision. All in all, then, the work of Anaxagoras must stand as that of perhaps the most far-seeing scientific imagination of pre-Socratic antiquity.