But while the Stoics regarded our universe as an island of being in an ocean of void, they did not admit the possibility that other such islands might exist beyond our ken. The spectacle of the starry heavens, which presented itself nightly to their gaze in all the brilliancy of a southern sky—that was all there was of being, beyond that lay nothingness. Democritus or the Epicureans might dream of other worlds, but the Stoics contended for the unity of the cosmos as staunchly as the Mahometans for the unity of God, for with them the cosmos was God.
In shape they conceived of it as spherical, on the ground that the sphere was the perfect figure and was also the best adapted for motion. Not that the universe as a whole moved. The earth lay in its centre, spherical and motionless, and round it coursed the sun, moon, and planets, fixed each in its respective sphere as in so many concentric rings, while the outermost ring of all, which contained the fixed stars, wheeled round the rest with an inconceivable velocity.
The tendency of all things in the universe to the centre kept the earth fixed in the middle as being subject to an equal pressure on every side. The same cause also, according to Zeno, kept the universe itself at rest in the void. But in an infinite void, it could make no difference whether the whole were at rest or in motion. It may have been a desire to escape the notion of a migratory whole which led Zeno to broach the curious doctrine that the universe has no weight, as being composed of elements whereof two are heavy and two are light. Air and fire did indeed tend to the centre like everything else in the cosmos, but not till they had reached their natural home. Till then they were of an upward-growing nature. It appears then that the upward and downward tendencies of the elements were held to neutralise one another and to leave the universe devoid of weight.
The universe was the only thing which was perfect in itself, the one thing which was an end in itself. All other things were perfect indeed as parts, when considered with reference to the whole, but were none of them ends in themselves, unless man could be deemed so who was born to contemplate the universe and imitate its perfections. Thus, then, did the Stoics envisage the universe on its physical side—as one, finite, fixed in space, but revolving round its own centre, earth, beautiful beyond all things, and perfect as a whole.
But it was impossible for this order and beauty to exist without mind. The universe was pervaded by intelligence as man's body is pervaded by his soul. But as the human soul though everywhere present in the body is not present everywhere in the same degree, so it was with the world-soul. The human soul presents itself not only as intellect, but also in the lower manifestations of sense, growth, and cohesion. It is the soul which is the cause of the plant life, which displays itself more particularly in the nails and hair; it is the soul also which causes cohesion among the parts of the solid substances such as bones and sinews, that make up our frame. In the same way the world-soul displayed itself in rational beings as intellect, in the lower animals as mere souls, in plants as nature or growth, and in inorganic substances as 'holding' or cohesion. To this lowest stage add change, and you have growth or plant nature; super-add to this phantasy and impulse and you rise to the soul of irrational animals; at a yet higher stage you reach the rational and discursive intellect, which is peculiar to man among mortal natures.
We have spoken of soul as the cause of the plant life in our bodies, but plants were not admitted by the Stoics to be possessed of soul in the strict sense. What animated them was 'nature' or, as we have called it above, 'growth'. Nature, in this sense of the principle of growth, was defined by the Stoics as 'a constructive fire, proceeding in a regular way to production,' or 'a fiery spirit endowed with artistic skill'. That Nature was an artist needed no proof, since it was her handiwork that human art essayed to copy. But she was an artist who combined the useful with the pleasant, aiming at once at beauty and convenience. In the widest sense, Nature was another name for Providence, or the principle which held the universe together, but, as the term is now being employed, it stood for that degree of existence which is above cohesion and below soul. From this point of view, it was defined as "a cohesion subject to self originated change in accordance with seminal reasons effecting and maintaining its results in definite times, and reproducing in the offspring the characteristics of the parent". This sounds about as abstract as Herbert Spencer's definition of life, but it must be borne in mind that nature was all the time a 'spirit', and as such a body. It was a body of a less subtle essence than soul. Similarly, when the Stoics spoke of cohesion, they are not to be taken as referring to some abstract principle like attraction. 'Cohesions,' said Chrysippus, 'are nothing else than airs, for it is by these that bodies are held together, and of the individual qualities of things which are held together by cohesion, it is the air which is the compressing cause which in iron is called "hardness", in stone "thickness" and in solver "whiteness"'. Not only solidarity then, but also colours, which Zeno called 'the first schematisms' of matter were regarded as due to the mysterious agency of air. In fact, qualities in general were but blasts and tensions of the air, which gave form and figure to the inert matter underlying them.
As the man is in one sense the soul, in another the body, and in a third the union of both, so it was with the cosmos. The word was used in three senses—
(1) God
(2) the arrangement of the stars, etc.
(3) the combination of both.
The cosmos as identical with God was described as an individual made up of all being who is incorruptible and ungenerated, the fashioner of the ordered frame of the universe, who at certain periods of time absorbs all being into himself and again generates it from himself. Thus the cosmos on its external side was doomed to perish and the mode of its destruction was to be by fire, a doctrine which has been stamped upon the world's belief down to the present day. What was to bring about this consummation was the soul of the universe becoming too big for its body, which it would eventually swallow up altogether. In the efflagration, when everything went back to the primeval aether, the universe would be pure soul and alive equally through and through. In this subtle and attenuated state, it would require more room than before and so expand into the void, contracting again when another period of cosmic generation had set in. Hence the Stoic definition of the Void or Infinite as that into which the cosmos is resolved at the efflagration.
In this theory of the contraction of the universe out of an ethereal state and ultimate return to the same condition one sees a resemblance to the modern scientific hypothesis of the origin of our planetary system out of the solar nebula, and its predestined end in the same. Especially is this the case with the form in which the theory was held by Cleanthes, who pictured the heavenly bodies as hastening to their own destruction by dashing themselves, like so many gigantic moths, into the sun. Cleanthes however did not conceive mere mechanical force to be at work in this matter. The grand apotheosis of suicide which he foresaw was a voluntary act; for the heavenly bodies were Gods and were willing to lose their own in a larger life.