Since, however, nature is a principle of motion and mutation, and the method with us is concerning nature, it is necessary that we should not be ignorant what motion is: for this being unknown, nature also must necessarily be unknown. And as we intend to define motion, we must also endeavor, after the same manner, to discuss those things which are consequent to this. But motion appears to belong to things continuous; and the infinite first presents itself to the view in that which is continuous. Hence it frequently happens that those who define the continuous, employ the nature of the infinite, as if that which is divisible to Infinity is continuous. To which we may add, that it is impossible for motion to subsist without place, and void, and time. It is evident, therefore, that for these reasons, and because these things are common and universal to all things, we should in the first place make each of these the object of consideration: for the speculation of peculiarities is posterior to that of things common. But first, as we have said, we must direct our attention to motion. There is then something which is in energy only; and there is something which is both in energy and capacity. One thing also is this particular thing; another so much in quantity; and another of a certain quality: and, in like manner, in the other categories of being. But of relatives, one is predicated according to excess and defect: another according to the effective and passive, and, in short, the motive, and that which may be moved: for the motive is motive of that which may be moved; and that which may be moved, may be moved by that which is motive. Motion, however, has not a subsistence separate from things: for that which is changed is always changed, either according to essence, or according to quantity, or according to quality, or according to place. But in these, as we have said, nothing common can be assumed which is neither this particular thing, nor a participant of quantity or quality, nor of any one of the other categories. Hence, there will not be either motion or mutation of anything except the particulars we have mentioned. But each of the categories subsists in a twofold manner in all things. Thus with respect to this particular thing, one thing pertaining to it is form, and another privation. In quality, one thing is white, and another black. And in quantity, one thing is perfect and another imperfect. In like manner also in relation, one thing is above, and another beneath; or one thing is light, and another heavy. So that the species of motion and mutation are as many as those of being. But since in every genus of things, there is that which is in entelecheia, and that which is incapacity; motion is the entelecheia of that which is incapacity, so far as it is incapacity. Thus change according to quality is the energy of that which may be so changed, so far as it is capable of this change: and increase and diminution are the energies of that which may be increased, and of its opposite, that which may be diminished; for there is no name common to both. Generation and corruption, also, are the energies of that which may be generated, and of that which may be corrupted: and lation is the energy of that which may be locally moved. But that this is motion is hence evident: for when that which may be built so far as we say it is such, is in energy, it is then built, and this is building. In like manner with respect to discipline, healing, rolling, and leaping, and a progression to manhood and old age. However, though some things are the same both in capacity and energy, yet not at the same time, or not according to the same, but, for instance, a thing may be hot in capacity, but cold in energy: and many things act upon and are passive to each other; for each of these will be at the same time effective and passive. Hence that which naturally moves is also movable; for everything of this kind moves, since it is also that which is moved. To some, therefore, it appears that everything which moves is moved. Whether, however, this be true or not, will be manifest from some other of our writings; for there is something which moves, and is itself immovable. But motion is the entelecheia of that which is in capacity, when being in entelechy area it energizes, not so far as it is that which it is, but so far as it is movable. I say so far as it is movable: for brass is incapacity a statue; yet, at the same time, motion is not the energy of the brass so far as it is brass; since the essence of brass is not the same as something which is movable in capacity: for if they were simply the same, and according to definition, motion would be the energy of the brass, so far as it is brass. They are, however, as we have said, not the same. And this is evident in contraries: for that which has the capacity of being well, and that which has the capacity of being diseased, differ from each other; (since otherwise to be deceased would be the same thing as to be well) but the subject, that which is well, and that which is diseased, whether it be humour or blood, are one and the same. But since that which is incapacity is not the same, according to definition, as that which is in energy, as neither is color the same as that which is visible; it is evident that motion is the entelecheia of that which is incapacity, so far as it is incapacity. That there is this energy, therefore, and that a thing then happens to be moved, when this energy exists, and neither prior nor posterior to it, is manifest. For everything is able at one time to energize, and add another not; as, for instance, that which is capable of being built, so far as it is capable. And the energy of that which is capable of being built, so far as it is thus capable, is building: for building is either this, the energy of that which is capable of being built, or it is the house. But when the house is, it will no longer be that which is capable of being built. That however is built, which is capable of being built; and hence it is necessary that building should be the above mentioned energy. But building is a certain motion. And the same reasoning will apply to other motions. That what we have asserted, too, is well said, is also evident from what others have said concerning motion, and from it not being easy to define it otherwise: for neither motion nor mutation can be placed in any other genus; nor have those who have advanced a different opinion concerning it spoken rightly. But this will be evident to those who consider the affair; for by some motion is said to be difference, inequality, and non-being; though it is not necessary that any one of these should be moved, neither if they be different, nor if they be unequal, nor if they be non-beings. Neither is mutation into these, nor from these, rather than from their opposites. But the reason why they referred motion to these things is this, that motion appears to be something indefinite; and the principles of the other coordination, are indefinite, because they are privative: for no one of them is this particular thing, nor of a particular quality, nor does it belong to any of the other categories. The cause, however, why motion appears to be indefinite, is because it can neither be simply referred to the capacity, nor to the energy of beings. For neither that which has the capacity of being a definite quantity is moved from necessity, nor that which is a definite quantity in energy. Motion, two, appears to be a certain energy, but imperfect; the cause of which is, that the capacity of that is imperfect, of which the energy is motion. On this account it is difficult to apprehend what motion is: for it is necessary to refer it either to privation, or to capacity, or to simple energy; but it does not appear that it can be any one of these. The above mentioned mode, therefore, remains, namely, that it is a certain energy; but such an energy as we have said it is, difficult indeed to be perceived, but which may have a subsistence.
