"Necessity then requires earth, as the element standing still at the centre. Now if there must be earth, there must be fire. For if one of two opposites is natural or necessary, the other must be necessary too, each, in fact, implying the necessity of the other. For the two have the same substantial basis, only the positive form is naturally prior to the negative; for instance, warm is prior to cold. And in the same way motionlessness and heaviness are predicated in virtue of the absence of motion and lightness, _i.e._ the latter are essentially prior.
"Further, if there are fire and earth, there must also be the elements which lie between these, each having an antithetic relation to each. From this it follows that there must be a process of coming into being, because none of these elements can be eternal, { 202} but each affects, and is affected by each, and they are mutually destructive. Now it is not to be argued that anything which can be moved can be eternal, except in the case of that which by its own nature has eternal motion. And if coming into being must be predicated of these, then other forms of change can also be predicated" (Arist. _De Coelo_, ii. p. 3).
This passage is worth quoting as illustrating, not only Aristotle's conception of the divine entelechy, but also the ingenuity with which he gave that appearance of logical completeness to the vague and ill-digested scientific imaginations of the time, which remained so evil an inheritance for thousands of years. It is to be observed, in order to complete Aristotle's theory on this subject, that the four elements, Earth, Water, Air, Fire, are all equally in a world which is "contrary to nature," that is, the world of change, of coming into being, and going out of being. Apart from these there is the element of the Eternal Cosmos, which is "in accordance with nature," having its own natural and eternal motion ever the same. This is the fifth or divine element, the aetherial, by the schoolmen translated _Quinta Essentia_, whence by a curious degradation we have our modern word Quintessence, of that which is the finest and subtlest extract.
Still more clearly is the organic conception carried { 203} out in Aristotle's discussion of the Vital principle or Soul in the various grades of living creatures and in man. It will be sufficient to quote at length a chapter of Aristotle's treatise on the subject (_De Anima_, ii. p. 1) in which this fundamental conception of Aristotle's philosophy is very completely illustrated:--