Again, it is requisite to define what kind of motion is contrary to motion; and after the same manner concerning permanency. But, in the first place, we must determine whether the motion which is from the same, is contrary to that motion which is to the same; as for instance, whether the motion which is from health, is contrary to that which tends to health, as appears to be the case with generation and corruption; or whether this is true of the motion which is from contraries, so that the motion, which is from health, is contrary to that which is from disease; or if this is the case with the motion that tends to contraries, so that the motion tending to health is contrary to that which tends to disease; or if the motion from a contrary to a contrary, is contrary to that from a contrary to a contrary; as for instance, if the motion from health to disease, is contrary to the motion from disease to health: for it is necessary: that there should either be one, or many of these modes; since it is not possible otherwise to oppose. But, the motion which is from a contrary, is not contrary to that which tends to a contrary. Thus the motion from health is not contrary to the motion to disease; for it is one and the same. They have not, however, the same essence; as neither is it the same thing to be changed from health, and to be changed into disease. Nor is the motion from one contrary, contrary to that which is from another: for at the same time it would happen that the motion would be from a contrary, and to a contrary, or to that which is intermediate. Concerning this, however, we shall speak hereafter. But the being changed to a contrary may appear to be the cause of contrariety, rather than the being changed from a contrary: for the one is a liberation from, but the other an assumption of, contrariety. And every motion is denominated rather from that into which it is changed, than from that from which it is changed. Thus that is called a becoming well (hygiansis) which tends to health, but a becomiag ill (nosansis) which tends to disease. There remains, therefore, the motion which is to contraries, and that which is to contraries from contraries. Perhaps, therefore, it happens that the motions which are to contraries, are also from contraries; but their essence, perhaps, is not the same. I mean, for instance, the motion to health, when referred to that which is from disease, and the motion from health referred to that which tends to disease. But since mutation differs from motion, for the mutation from a certain subject to a certain subject, is motion; this being the case, the motion which is from a contrary to a contrary, is contrary to that which is from a contrary to a contrary. Thus the motion which is from health to disease, is contrary to the motion from disease to health. But it is manifest from induction, what kind of things those appear to be which are contraries: for to become diseased is contrary to the being made well; and to learn is contrary to the being deceived, not through oneself; smce they tend to contraries: for as science, so likewise deception may be obtained, both through oneself and through another. Lation, upward also, is contraty to that which is downward; for these are contrary in length. And the motion to the right hand, is contrary to. that which is to the left; for these are contrary in breadth.. The motion too, which tends to the anterior part, contrary to that which tends to the posterior part: for these are contrary in depth. But the tendency, which is to a contrary alone, is not motion, but mutation; such, for instance, as becoming to be white, but not from a certain thing. In things, however, to which there is not any thing contrary, the mutation which is from the same, is contrary to the mutation which ts to the same. Hence generation is contrary to corruption, and rejection to assumption. But these are mutations, and not motions. The motions, however, to that which has an intermediate subsistence, between which being contraries something intervenes, are to be considered in a certain respect as tending to contraries; for motion uses that which is intermediate as a contrary, into which ever part it may be changed; for instance, from a dark brown to white, as from black; and from white to a dark brown, as to black; but from black to a dark brown, as to white: for the dark brown which is a medium, is, in a certain respect; referred to each of the extremes, as was before observed. Motion, therefore, is thus contrary to motion; viz. the motion from a contrary to a contrary, is contrary to that from a contrary to a contrary.
