The difficulty, however, vanishes when we understand the function ascribed by Plato to the earth as I have endeavoured to elucidate it. The earth not only is not at rest, but cannot be at rest, precisely because it is packed round the solid revolving cosmical axis, and must revolve along with it. The function of the earth, as the first and oldest of intra-kosmic deities, is to uphold and regulate the revolutions of this axis, upon which depend the revolutions of the sidereal sphere or outer sh.e.l.l of the kosmos.
It is by virtue of this regulating function (and not by virtue of its rotation) that the earth is the guardian and artificer of night and day. It is not only "an instrument of time," but the most potent and commanding among all instruments of time.
What has just been stated is, in my belief, the theory of the Platonic Timaeus, signified in the words of that dialogue, and embodied in the comment of Aristotle. The commentators, subsequent to Aristotle, so far as we know them, understood the theory in a sense different from what Plato intended. I think we may see how this misconception arose. It arose from the great development and elaboration of astronomical theory during the two or three generations immediately succeeding Plato. Much was added by Eudoxus and others, in their theory of concentric spheres: more still by others of whom we read in Cicero (Academ. II. 39.) "Hicetas Syracusius, ut ait Theophrastus, coelum, solem, lunam, stellas, supera denique omnia, stare censet, neque praeter terram rem ullam in mundo moveri: quae c.u.m circ.u.m axem se summa celeritate convertat et torqueat, eadem effici omnia, quae si stante terra coelum moveretur. Atque hoc etiam Platonem in Timaeo dicere quidam arbitrantur, sed paullo obscurius." The same doctrine is said to have been held by Herakleides of Pontus, the contemporary of Aristotle, and by others along with him.
(Simplikius ad Aristot. Physic. p. 64--De Coelo, p. 132--Plutarch.
Plac. Phil. III. 13.) The doctrine of the rotation of the Earth here appears along with another doctrine--the immobility of the sidereal sphere and of the celestial bodies. The two are presented together, as correlative portions of one and the same astronomical theory. There are no celestial revolutions, and therefore there is no solid celestial axis. Moreover, even Aristarchus of Samos (who attained to a theory substantially the same as the Copernican, with the double movement of the Earth, rotation round its own axis, and translation round the sun as a centre) comes within less than a century after Plato"s death.
Though the _quidam_ alluded to by Cicero looked upon the obscure sentence in Plato"s Timaeus as a dim indication of the theory of Hicetas, yet the two agree only in the supposition of a rotation of the earth, and differ essentially in the pervading cosmical conceptions. Hicetas states distinctly that which his theory denies, as well as that which it affirms. The negation of the celestial rotations, is in his theory a point of capital and coordinate importance, on which he contradicts both Plato and Aristotle as well as the apparent evidence of sense. I cannot suppose that this theory can have been proclaimed or known to Aristotle when his works were composed: for the celestial revolutions are the keystone of his system, and he could hardly have abstained from combating a doctrine which denied them altogether. In the hands of Hicetas (perhaps in those of Herakleides, if we may believe what is said about him) astronomy appears treated as a science by itself, with a view "to provide such hypotheses as may save the phenomena" ([Greek: so/zein ta phaino/mena], Simpl. ad Aristot. De Coelo, p. 498, Schol.
Brandis). It becomes detached from those religious, ethical, poetical, teleological, arithmetical decrees or fancies, in which we see it immersed in the Platonic Timaeus, and even (though somewhat less) in the Aristotelian Treatise De Coelo. Hence the meaning of Plato, obscurely announced from the beginning, ceased to be understood: the solid revolving axis of the Kosmos, a.s.sumed without being expressly affirmed in his Timaeus, dropped out of sight: the doctrine of the rotation of the earth was presented in a new point of view, as a subst.i.tute for the celestial revolutions. But no proper note was taken of this transition. The doctrine of Plato was a.s.sumed to be the same as that of Hicetas.
When we read Plutarch"s criticism (Quaest. Plat. p. 1006 C) upon the word [Greek: i)llome/nen], we see that he puts to himself the question thus--"Does Plato in the Timaeus conceive the earth as kept together and stationary--or as turning round and revolving, agreeably to the subsequent theory of Aristarchus and Seleukus?"
Here we find that Plutarch conceives the alternative thus--Either the earth does not revolve at all, or it revolves as Aristarchus understood it. One or other of these two positions must have been laid down by Plato in the Timaeus.--So we read in Plutarch. But the fact is, that Plato meant neither the one nor the other. The rotation of the earth round the solid cosmical axis, which he affirms in the Timaeus--is a phenomenon utterly different from the rotation of the earth as a free body round the imaginary line called its own axis, which was the doctrine of Aristarchus.
When expositors in Plutarch"s day, and since his day, enquired whether or not the Platonic Timaeus affirmed the rotation of the earth, they meant to designate the rotation of the earth in the sense of Aristarchus, and in the sense in which modern astronomy understands that capital fact. Now speaking the language of modern astronomy, I think it certain that the rotation of the earth is _not_ to be found affirmed in the Platonic Timaeus; and I agree with M. Boeckh when he says (Untersuch. p. 77), "Granting that Aristotle ascribed to Plato the doctrine of the rotation of the earth, he at least did not ascribe to him the doctrine as Gruppe a.s.sumes, and as now understood." As between Gruppe--who holds that the Platonic Timaeus affirms the rotation of the earth, and that Aristotle ascribes it to him, in our sense of the words--and M. Boeckh, who denies this--I stand with the latter for the negative. But when M. Boeckh a.s.sumes that the only alternative doctrine is the immobility of the earth, and tries to show that this doctrine is proclaimed in the Platonic Timaeus--nay, that no opposite doctrine _can_ be proclaimed, because the discourse expressly announces the rotation of the sidereal heaven in twenty-four hours--I am compelled to dissent from him as to the conclusion, and to deny the cogency of his proof. M. Boeckh has hardly asked himself the question, whether there was not some other sense in which Plato might have affirmed it in the Timaeus. I have endeavoured to show that there was another sense; that there are good a.n.a.logies in Plato to justify the belief that he intended to affirm the doctrine in that other sense; and that the comments of Aristotle--while thoroughly pertinent, if we thus understand the pa.s.sage in the Timaeus--become either irrelevant, dishonest, or absurd, if we construe the pa.s.sage as signifying either what is maintained by M. Boeckh or what is maintained by Gruppe.
The eminent critics, whose opinions I here controvert, have been apparently misled by the superior astronomical acquirements of the present age, and have too hastily made the intellectual exigencies of their own minds a standard for all other minds, in different ages as well as in different states of cultivation. The question before us is, not what doctrines are scientifically true or scientifically compatible with each other, but what doctrines were affirmed or implied by Plato. In interpreting him, we are required to keep our minds independent of subsequent astronomical theories. We must look, first and chiefly, to what is said by Plato himself; next, if that be obscure, to the construction and comments of his contemporaries so far as they are before us. In no case is this more essential than in the doctrine of the rotation of the earth, which in the modern mind has risen to its proper rank in scientific importance, and has become connected with collateral consequences and a.s.sociations foreign to the ideas of the ancient Pythagoreans, or Plato, or Aristotle. Unless we disengage ourselves from these more recent a.s.sociations, we cannot properly understand the doctrine as it stands in the Platonic Timaeus.
This doctrine, as I have endeavoured to explain it, leads to an instructive contrast between the cosmical theories of Plato (in the Timaeus) and Aristotle.
Plato conceives the kosmos as one animated and intelligent being or G.o.d, composed of body and soul. Its body is moved and governed by its soul, which is fixed or rooted in the centre, but stretches to the circ.u.mference on all sides, as well as all round the exterior. It has a perpetual movement of circular rotation in the same unchanged place, which is the sort of movement most worthy of a rational and intelligent being. The revolutions of the exterior or sidereal sphere (Circle of the Same) depend on and are determined by the revolutions of the solid cylinder or axis, which traverses the kosmos in its whole diameter. Besides these, there are various interior spheres or circles (Circles of the Different), which rotate by distinct and variable impulses in a direction opposite to the sidereal sphere. This latter is so much more powerful than they, that it carries them all round with it; yet they make good, to a certain extent, their own special opposite movement, which causes their positions to be ever changing, and the whole system to be complicated. But the grand capital, uniform, overpowering, movement of the kosmos, consists in the revolution of the solid axis, which determines that of the exterior sidereal sphere. The impulse or stimulus to this movement comes from the cosmical soul, which has its root in the centre.
Just at this point is situated the earth, "the oldest and most venerable of intra-kosmic deities," packed round the centre of the axis, and having for its function to guard and regulate those revolutions of the axis, and through them those of the outer sphere, on which the succession of day and night depends--as well as to nurse mankind.
In all this we see that the ruling principle and force of the kosmos ([Greek: to e(gemonikon tou= ko/smou]) is made to dwell in and emanate from _its centre_.
When we come to Aristotle, we find that the ruling principle or force of the kosmos is placed, not in its centre, but in its circ.u.mference. He recognises no solid revolving axis traversing the whole diameter of the kosmos The interior of the kosmos is occupied by the four elements--earth, water, air, fire--neither of which can revolve except by violence or under the pressure of extraneous force. To each of them rectilinear motion is _natural_; earth moves naturally towards the centre--fire moves naturally towards the circ.u.mference, away from the centre.
But the peripheral substance of the kosmos is radically distinct from the four elements: rotatory motion in a circle is _natural_ to it, and is the only variety of motion natural to it. That it is moved at all, it owes to a _primum movens immobile_ impelling it: but the two are coeternal, and the motion has neither beginning nor end. That when moved, its motion is rotatory and not rectilinear, it owes to its own nature. It rotates perpetually, through its own nature and inherent virtue, not by constraining pressure communicated from a centre or from a soul. If constraint were required--if there were any contrary tendency to be overcome--the revolving periphery would become fatigued, and would require periods of repose; but, since in revolving it only obeys its own peculiar nature, it persists for ever without knowing fatigue. This peripheral or fifth essence, perpetually revolving, is the divine, venerable, and commanding portion of the kosmos, more grand and honourable than the interior parts or the centre. Aristotle lays this down (De Coelo, ii. 13, p. 293, b. 10) in express ant.i.thesis to the Pythagoreans, who (like Plato) considered the centre as the point of grandeur and command, placing fire in the centre for that reason. The earth has no positive cosmical function in Aristotle; it occupies the centre because all its parts have a natural movement towards the centre: and it is unmoved because there _must be_ something in the centre which is always stationary, as a contrary or ant.i.thesis to the fifth essence or peripheral substance of the kosmos, which is in perpetual rotation by its own immutable nature.
I do not here go farther into the exposition of these ancient cosmical theories. I have adverted to Aristotle"s doctrine only so far as was necessary to elucidate, by contrast, that which I believe to be the meaning of the Platonic Timaeus about the rotation of the earth.