192. In including in their system the study of the physical universe the Stoics broke daringly with Socrates and his faithful followers the Cynics. These had joined with the ignorant and the prejudiced[1] in ridiculing those whose eyes were always turned up towards the sky, whilst they saw nothing of things that were nearer at hand and concerned them more closely. But it was not for nothing that the most highly civilised nations of antiquity, Egyptians, Chaldaeans, and Babylonians, had studied the starry heavens, mapped out the constellations, measured the paths of the wandering stars, predicted eclipses, reckoned with the tides, the seasons, and the winds; with the result that their successors defied the common opinion by declaring the earth to be a sphere, and to hold inhabitants whom they called Antipodes, because they walk with their feet turned up towards ours[2]. All this body of knowledge, called generically the knowledge of the sky (though it included the whole physical geography of the earth), had impressed and fascinated the Eastern world. It seemed that as the eyes were raised to the sky, so the mind of man was elevated and made ampler and nobler[3], leaving behind it the petty contentions and rivalries of common life; and further that true knowledge had surely been reached, when the positions of the heavenly bodies and the eclipses of sun and moon could be predicted so long before with unfailing accuracy. These feelings are now commonplaces of literature, and were fully shared by the Stoics. ‘Is not the sun,’ says Seneca, ‘worthy of our gaze, the moon of our regard? When the sky displays its fires at night, and countless stars flash forth, who is not absorbed in contemplation of them? They glide past in their company, concealing swift motion under the outward appearance of immobility. We comprehend the movements of a few of them, but the greater number are beyond our ken. Their dignity fills all our thoughts[4].’ In the golden age which preceded our iron civilisation ‘men lay at nights in the open fields, and watched the glorious spectacle of the heavens. It was their delight to note the stars that sank in one quarter and rose in another. The universe swept round them, performing its magnificent task in silence[5].’ ‘Their order never changes, spring and autumn, winter and summer succeed according to fixed laws[6].’ And in the same tone writes the Stoic poet: ‘unshaken the lights of heaven ever move onwards in their proper orbit[7].’ The emotion roused in the Stoic by the contemplation of the sky was thus identical with that expressed in Judaic poetry by the ‘Song of the Three Holy Children[8],’ and in more modern times by Addison’s famous hymn[9].
The world-order.
193. The phenomena of earth and heaven combined, in the general opinion of intelligent men, to show the existence of a ‘world-order’ or ‘universe[10].’ The Stoics accepted this conception in their physics from Heraclitus, who had declared that ‘neither god nor man created this world-order,’ as in their ethics from Diogenes, the ‘citizen of the universe[11].’ They therefore needed only to adjust an established notion to their own physical postulates. We observe at once that the very conception of an ordered whole differentiates that whole from the absolute totality of all things. The universe is indeed on the one hand identified with the substance of all things (οὐσία τῶν ὅλων), but only as a thing made individual by the possession of quality (ἰδίως ποιόν)[12], and necessarily one[13]. It is self-created; and it may therefore be identified with its creator, the deity[14]; it also includes all that is bodily[15]; but outside there remains the boundless void[16]. It is therefore defined by Chrysippus as ‘the combination of heaven and earth and all natures that are in them,’ or alternatively as ‘the combination of gods and men and all that is created for their sake[17].’
Its position.
194. The Stoic conception of the universe is therefore that of a continuous body, having a definite outline, and stationed in the boundless void. That the universe has shape the Stoics deduce from its having ‘nature’ (φύσις), that is, the principle of growth, displayed in the symmetry of its parts[18]; and its shape is the perfect shape of a sphere[19]. Within this sphere all things tend towards the middle[20]; and we use the terms ‘down’ meaning ‘towards the middle,’ and ‘up’ meaning thereby from the middle[21]. The Peripatetics are therefore needlessly alarmed, when they tell us that our universe will fall down, if it stands in the void; for, first, there is no ‘up’ or ‘down’ outside the universe; and, secondly, the universe possesses ‘unity’ (ἕξις)[22] which keeps it together[23]. And here we see the folly of Epicurus, who says that the atoms move downwards from eternity in the boundless void; for there is no such thing as ‘downwards’ in that which is unlimited[24]. Further, the universe is divided into two parts, the earth (with the water and the air surrounding it) which is stable in the middle, and the sky or aether which revolves around it[25].
The heliocentric theory.
195. Thus early in their theory the Stoics were led to make two assertions on questions of scientific fact, in which they opposed the best scientific opinion of their own time. For many authorities held that the earth revolved on its axis, and that the revolution of the sky was only apparent. Such were Hicetas of Syracuse[26], a Pythagorean philosopher, whose views were quoted with approval by Theophrastus, and later Ecphantus the Pythagorean, and Heraclides of Pontus[27]. From the point of view of astronomical science this view seemed well worthy of consideration, as Seneca in particular emphasizes[28]. Other astronomers had gone further, declaring that the sun lay in the centre, and that the earth and other planets revolved round it. Theophrastus stated that Plato himself in his old age had felt regret that he had wrongly placed the earth in the centre of the universe; and the heliocentric view was put forward tentatively by Aristarchus of Samos, and positively by the astronomer Seleucus, in connexion with the theory of the earth’s rotation[29]. For this Cleanthes had said that the Greeks should have put Aristarchus on trial for impiety, as one who proposed to disturb ‘the hearth of the universe[30].’ This outburst of persecuting zeal, anticipating so remarkably the persecution of Galileo, was effective in preventing the spread of the novel doctrine. Posidonius was a great astronomer, and recognised the heliocentric doctrine as theoretically possible[31]; indeed, as one who had himself constructed an orrery, shewing the motion of all the planets[32], he must have been aware of its superior simplicity. Nevertheless he opposed it vigorously on theological grounds, and perhaps more than any other man was responsible for its being pushed aside for some 1500 years[33]. The precise ground of the objection is not made very clear to us, and probably it was instinctive rather than reasoned. It could hardly be deemed impious to place the sun, whom the Stoics acknowledged as a deity, in the centre of the universe; but that the earth should be reckoned merely as one of his attendant planets was humiliating to human self-esteem, and jeopardised the doctrine of Providence, in accordance with which the universe was created for the happiness of gods and men only.
The elements.
196. Having determined that the earth is the centre of the universe, and the sun above it, the way is clear to incorporate in the system the doctrine of the four elements (στοιχεῖα, naturae)[34], which probably had its origin in a cruder form of physical speculation than the doctrine of the heavenly bodies. As we have seen above[35], the elements are not first principles of the Stoic physics, but hold an intermediate position between the two principles of the active and the passive on the one hand, and the organic and inorganic world on the other. Earth is the lowest of the elements, and also the grossest; above it is placed water, then air, then fire; and these are in constant interchange, earth turning to water, this into air, and this into aether, and so again in return. By this interchange the unity of the universe is maintained[36]. The transition from one element to the next is not abrupt, but gradual; the lowest part of the aether is akin to air[37]; it is therefore of no great importance whether we speak with Heraclitus of three elements, or with Empedocles of four. The two grosser elements, earth and water, tend by nature downwards and are passive; air and fire tend upwards and are active[38]. Zeno did not think it necessary to postulate a fifth element as the substance of soul, for he held that fire was its substance[39].
Fire and breath.