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.