Throughout Aristotle's physical philosophy the [334] same conception runs: "All animals in their fully developed state require two members above all--one whereby to take in nourishment, the other whereby to get rid of what is superfluous. For no animal can exist or grow without nourishment. And there is a third member in them all half-way between these, in which resides the principle of their life. This is the heart, which all blood-possessing animals have. From it comes the arterial system which Nature has made hollow to contain the liquid blood. The situation of the heart is a commanding one, being near the middle and rather above than below, and rather towards the front than the back. For Nature ever establishes that which is most honourable in the most honourable places, unless some supreme necessity overrules. We see this most clearly in the case of man; but the same tendency for the heart to occupy the centre is seen also in {200} other animals, when we regard only that portion of their body which is essential, and the limit of this is at the place where superfluities are removed. The limbs are arranged differently in different animals, and are not among the parts essential to life; consequently animals may live even if these are removed. . . . Anaxagoras says that man is the wisest of animals because he possesses hands. It would be more reasonable to say that he possesses hands because he is the wisest. For the hands are an instrument; and Nature always assigns an instrument to the one fitted to use it, just as a sensible man would. For it is more reasonable to give a flute to a flute-player than to confer on a man who has some flutes the art of playing them. To that which is the greater and higher she adds what is less important, and not _vice versa_. Therefore to the creature fitted to acquire the largest number of skills Nature assigned the hand, the instrument useful for the largest number of purposes" (Arist. _De Part. An._ iv. p. 10). [332] And in the macrocosm, the visible and invisible world about us, the same conception holds: "The existence of God is an eternally perfect entelechy, a life everlasting. In that, therefore, which belongs to the divine there must be an eternally perfect movement. Therefore the heavens, which are as it were the body of the Divine, are in form a sphere, of {201} necessity ever in circular motion. Why then is not this true of every portion of the universe? Because there must of necessity be a point of rest of the circling body at the centre. Yet the circling body cannot rest either as a whole or as regards any part of it, otherwise its motion could not be eternal, which by nature it is. Now that which is a violation of nature cannot be eternal, but the violation is posterior to that which is in accordance with nature, and thus the unnatural is a kind of displacement or degeneracy from the natural, taking the form of a coming into being.