But Aristotle argues that there must be a fifth element: and our modern word quintessence is, by the way, a relic of this argument, this fifth element having been called by later writers _quinta essentia_ or quintessence. The argument is as follows: "the simple elements must have simple motions, and thus fire and air have their natural motions upwards and water and earth have their natural motions downwards; but besides these motions there is motion in a circle which is unnatural to these elements, but which is a more perfect motion than the other, because a circle is a perfect line and a straight line is not; and there must be something to which this motion is natural. From this it is evident that there is some essence or body different from those of the four elements, ... and superior to them. If things which move in a circle move contrary to nature it is marvelous, or rather absurd that this, the unnatural motion should alone be continuous and eternal; for unnatural motions decay speedily. And so from all this we must collect that besides the four elements which we have here and about us, there is another removed far off and the more excellent in proportion as it is more distant from us." Or take Aristotle's dealing with the heaviness and lightness of bodies. After censuring former writers for considering these as merely relative, he declares that lightness is a positive or absolute property of bodies just as weight is; that earth is absolutely heavy, and therefore tends to take its place below the other three elements; that fire has the positive property of lightness, and hence tends to take its place above the other three elements; (the modern word _empyrean_ is a relic of this idea from the _pyr_ or fire, thus collected in the upper regions), and so on; and concludes that bodies which have the heavy property tend to the centre, while those with the light property tend to the exterior, of the earth, because "Exterior is opposite to Centre, as heavy is to light." This conception, or rather misconception, of opposites appears most curiously in two of the proofs which Socrates offers for the immortality of the soul, and I do not know how I can better illustrate the infirmity of antique thought which I have just been describing than by citing the arguments of Socrates in that connection according to the _Phædo_. Socrates introduced it with special solemnity. "I do not imagine," he says, "that any one, not even if he were a comic poet, would now say that I am trifling.... Let us examine it in this point of view, whether the souls of the dead survive or not. "Let us consider this, whether it is absolutely necessary in the case of as many things as have a contrary, that this contrary should arise from no other source than from a contrary to itself. For instance, where anything becomes greater, must it not follow that from being previously less it subsequently became greater? "Yes."