Aristotle twice[588] makes the statement that, though Empedokles assumes four elements, he treats them as two, opposing Fire to all the rest. This, he says, we can see for ourselves from his poem. So far as the general theory of the elements goes, it is impossible to see anything of the sort; but, when we come to the origin of the world (§ 112), we shall find that Fire certainly plays a leading part, and this may be what Aristotle meant. It is also true that in the biology (§ 114–116) Fire fulfils a unique function, while the other three act more or less in the same way. But we must remember that it has no pre-eminence over the rest: all are equal. Footnote 588: Arist. _Met._ Α, 4. 985 a 31; _de Gen. Corr._ Β, 3. 330 b 19 (R. P. 164 e). [Sidenote: Strife and Love.] 108. The Eleatic criticism had made it necessary for subsequent thinkers to explain motion.[589] Empedokles starts, as we have seen, from an original state of the “four roots,” which only differs from the Sphere of Parmenides in so far as it is a mixture, not a homogeneous and continuous mass. The fact that it is a mixture makes change and motion possible; but, were there nothing outside the Sphere which could enter in, like the Pythagorean “Air,” to separate the four elements, nothing could ever arise from it. Empedokles accordingly assumed the existence of such a substance, and he gave it the name of Strife. But the effect of this would be to separate all the elements in the Sphere completely, and then nothing more could possibly happen; something else was needed to bring the elements together again. This Empedokles found in Love, which he regarded as the same impulse to union that is implanted in human bodies (fr. 17, 22 sqq.). He looks at it, in fact, from a purely physiological point of view, as was natural for the founder of a medical school. No mortal had yet marked, he says, that the very same Love which men know in their bodies had a place among the elements. Footnote 589: Cf. Introd. § VIII. It is important to observe that the Love and Strife of Empedokles are no incorporeal forces, but corporeal elements like the other four. At the time, this was inevitable; nothing incorporeal had yet been dreamt of. Naturally, Aristotle is puzzled by this characteristic of what he regarded as efficient causes. “The Love of Empedokles,” he says[590] “is both an efficient cause, for it brings things together, and a material cause, for it is a part of the mixture.” And Theophrastos expressed the same idea by saying[591] that Empedokles sometimes gave an efficient power to Love and Strife, and sometimes put them on a level with the other four. The verses of Empedokles himself leave no room for doubt that the two were thought of as spatial and corporeal. All the six are called “equal.” Love is said to be “equal in length and breadth” to the others, and Strife is described as equal to each of them in weight (fr. 17). Footnote 590: Arist. _Met._ Α, 10. 1075 b 3.