Upon this disputed question, a feeling of perplexity comes unavoidably upon every one. It would argue a very irrational mind not to wonder how a small piece of the Earth, if suspended in the air, is carried downward and will not stop of itself, and the larger piece is carried downward more quickly than the smaller; while nevertheless the entire Earth, if suspended in like manner, would not be so carried. In spite of its great weight, it remains stationary (s. 6). But the solutions of this problem which some suggest are more strange and full of perplexity, and it is surprising that they have not been so considered. The Kolophonian Xenophanes affirmed that the lower depths of the Earth were rooted downwards to infinity, in order to escape the troublesome obligation of looking for a reason why it remained stationary. Others say, that the Earth rests upon water, floating thereupon like wood: this is an ancient doctrine promulgated by Thales; as if there were not as much perplexity about the water which supports the Earth, as there is about the Earth itself. For it is not the nature of water to remain suspended, but always to rest upon something (s. 7). Moreover, air is lighter than water, and water lighter than earth; how then can these men think that the substance naturally lighter can lie below the substance naturally heavier? Besides, if it were the nature of the whole Earth to remain resting on water, it must be the nature of each part of the Earth to do the same; but this does not happen: each part of the earth is carried down to the bottom, and the greater part more quickly than the less (s. 8).
All these philosophers carry their researches to a certain point, but not to the bottom of the problem. It is indeed a habit with all of us to conduct our enquiries not with reference to the problem itself, but with reference to our special opponents. If we have no opponent but are conducting our investigations alone, we pursue them as far as that point where we can make no farther objections to ourselves. Whoever therefore intends to investigate completely must take care to make objections to himself upon all the points of objection which really belong to the subject; and this he can only do after having thoroughly surveyed all the differences of opinion and doctrine (s. 9).
The reason why the Earth remains at rest, Anaximenes, Anaxagoras, and Demokritus, declare to be its breadth or flatness (τὸ πλάτος): it does not (they say) divide the air beneath, but covers over the air like a lid (οὐ γὰρ τέμνειν, ἀλλ’ ἐπιπωματίζειν τὸν ἀέρα τὸν κάτωθεν); as we see that flat and spreading bodies usually do, being difficult to be moved and making strong resistance even against the winds. The Earth does the same, through its flatness, against the air beneath, which remains at rest there (in the opinion of these philosophers) because it finds no sufficient place into which to travel, like water in a klepsydra: they also produce many evidences to show that air thus imprisoned, while remaining stationary, can support a heavy weight (s. 10).
Now, in the first place, these men affirm that, unless the shape of the Earth were flat, it would not remain at rest. Yet on their own showing it is not alone the flat shape of the earth which causes it to remain at rest, but rather its magnitude. For the air beneath remains in situ by reason of its vast mass, finding no means of escape through the narrow passage: and the mass of the air is thus vast, because it is imprisoned inside by the great magnitude of the Earth; which effect will be produced in the same manner, even though the Earth be spherical, provided it be of its present magnitude. Moreover, philosophers who hold this opinion about the motion of the Earth, think only of its motion as a whole, and take no account of its parts. For they ought to define at the first step whether bodies have or have not one special mode of motion by nature; and, if none by nature, then whether they have any mode of motion violent or contra-natural. I have already determined this point as well as my powers admitted, and shall therefore assume the results as settled. If there be no special motion natural to bodies, neither will there be any which is contra-natural or violent; and, if there be none either natural or violent, no body will be moved at all. I have already shown that this is a necessary consequence; and, farther, that (upon that supposition) there can be no body even at rest; for rest, like motion, is either natural or contra-natural; and, if there be any special mode of motion which is natural, neither contra-natural motion, nor contra-natural rest, can stand alone (ss. 11-13).
Let us then assume (reasoning on the hypothesis of these philosophers) that the Earth now remains in its present place contrary to nature, and that it was carried into aggregation at the centre by the revolution of the Kosmos (also contrary to nature — καὶ συνῆλθεν ἐπὶ τὸ μέσον φερομένη διὰ τὴν δίνησιν — s. 14). For all those who recognize a generation of the Kosmos assign this revolution as the cause which determined the aggregation of the Earth at the centre, upon the analogy of particles carried round in liquids or in air, where the larger and heavier particles are always carried to the centre of the revolution. They profess thus to know the cause which determined the Earth to come to the centre; but what they seek to find out is the cause which determines it to remain there, and upon that they differ: some saying, as has been stated just now, that its breadth and magnitude is the cause; others, with Empedokles, ascribing the fact to the revolution of the Heaven, the extreme velocity of which checks the fall of the Earth downward, just as water in a cup may be whirled rapidly round without falling to the ground. But suppose absence of these two causes: in which direction will the Earth be naturally carried? Not to the centre; for (upon the doctrine which we are now criticising) its motion to the centre, and its remaining at the centre, are both of them contra-natural; but some special mode of motion, natural to the Earth, there must necessarily be. Is this upward, or downward, or in what other direction? If there be no greater tendency downward than upward, and if the air above does not hinder the Earth from tending upward, neither will the air beneath hinder it from tending downwards: the same causes produce the same effects, operating on the same matter (ss. 14, 15).
A farther argument becomes applicable, when we are reasoning against Empedokles. When the four elements were first separated out of their confused huddle by the influence of Contention, what was the cause for the Earth to remain still and in situ? Empedokles cannot claim to introduce then the agency of the cosmical revolution. Moreover, it is strange that he should not have reflected that in the first instance the particles and fragments of the Earth were carried to the centre. But what is the cause now that every thing having weight is carried towards the Earth? It cannot be the revolution of the Heaven which brings these things nearer to us (s. 16).
Again, Fire is carried upward. What is the cause of this? The revolution of the Heaven cannot cause it. But, if it be the nature of fire to be carried in one certain direction, it must be equally the nature of Earth to be carried in one certain direction. Light and heavy, also, are not discriminated by the heavenly revolution. There are matters originally heavy, and matters originally light: the former are carried to the centre, the latter to the circumference, each by its own special motion. Even prior to the heavenly revolution there existed things intrinsically light and intrinsically heavy; which are discriminated by certain attributes — a certain natural mode of motion and a certain place. In infinite space, there can be no upward and downward; and it is by this (local distinction) that light and heavy are discriminated (ss. 17, 18).
While most philosophers insist upon the causes just noticed why the Earth remains stationary where it is, there are others, like Anaximander, among the ancients, who say that it remains so because of its likeness or equality (διὰ τὴν ὁμοιότερα — equal tendency in all directions). That which is situated in the centre (they say) and which has like relation to the extreme parts (i.e., like to all the extreme parts) ought not to be carried any more upward or downward or sideways; and it cannot be moved in opposite directions at once; so that it remains stationary by necessity (s. 19).
This doctrine is ingenious, but not true. For the property affirmed is noway peculiar to the Earth: the affirmation is, that every thing which is placed at the centre must of necessity remain there; so that Fire also would remain there at rest, as well as Earth. But this necessity must be denied. For it is shown by observation that the Earth not only remains at the centre, but is carried to the centre; since each part of it is carried thither, and, whithersoever the parts are carried, the whole is carried necessarily to the same point. The peculiar property of the earth therefore is, not (as this hypothesis declares) to have like relation to all the extreme parts — for that is common to all the elements — but to be carried towards the centre (ss. 20, 21).
Moreover, it is absurd to investigate why the Earth remains at the centre, and not to investigate equally why Fire remains at the extremity. For, if you explain this last by saying that Fire has its natural place at the extremity, the Earth must have its natural place somewhere else. If the centre be not the natural place of the Earth, and if the Earth remains there through like tendency in all directions, like the hair in equal tension or the man both hungry and thirsty between food and drink, you must equally assign the reason why Fire remains at the extremity. It is singular too that you should try to explain only the remaining at rest (μονῆς) of the Earth, and not also seek to explain the natural current (φορά) — why Earth is carried downward, and Fire upward, when there is no opposing force (s. 22).