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Whether motion is present in every animal.

Now one asks about the motion of animals. And first, whether motion is found in every animal.

1. It seems so. Motion occurs on account of some need. But every animal needs something, for example nourishment, which is often at some distance from it. Therefore, etc.

2. Again, motion follows sense. This is why Avicenna maintains that wherever sense is absent, there is no motion. Therefore, since every animal has sense, it will have motion.

3. Again, the cause of an animal's immobility is proximity to its food source [ coniunctio alimenti]. Conversely, therefore, the cause of mobility is separation from its food source. But every animal has a food source separated from it; therefore, etc.

The Philosopher says the opposite, and this is also obvious to the senses.

One must reply that locomotion is of two types: one is the motion of expansion and contraction, and the other is progressive motion. The first follows upon the sense of taste or touch and is present in every animal because there is no animal so immobile that it does not retreat as if it were pricked if something harmful approaches, whereas, if something agreeable approaches, it expands and spreads itself over it. The second type of motion is found only in those possessing sensations that take place by means of an external medium. For unless an animal has sight or smell or hearing, progressive motion may injure it more than assist it, since it might put itself in the way of things that corrupt as well as things that benefit. Therefore, since not every animal has sight, hearing, or smell, not every animal has progressive motion.

1. On to the arguments. To the first, one must reply that need is of two types: namely, for completion or for preservation. Now a human needs many things for his preservation, whereas immobile animals have a need for completion but do not need as many things for their preservation. And when one says that every animal needs nourishment that is at some distance from it, it must be said that although the nourishment of every animal is distinct substantially before its consumption, nevertheless in many cases it is in close proximity to it, and this is why motion is not needed for them to obtain it.

2. To the second, one must reply that motionwhether expansion or contraction or progressive motionfollows sensation. Those animals are called immobile that do not move progressively.

3. To the third, one must reply (and it is already clear) that the cause of motion is the separation of the food source with respect to substance and location. But the food source is not separated from every animal, as will be made clear.

Whether an immobile animal is always aquatic and not terrestrial.

Second, one asks whether an immobile animal is always aquatic and not terrestrial.

1. It seems not. The more form something has, the more activity [ operatio] it has. But water has more form than earth. Therefore, an aquatic animal has more form than a terrestrial animal and, as consequence, more activity. Therefore, if there is some aquatic animal that is immobile, a terrestrial one will be much more so.

2. Again, although plants are terrestrial they are immobile. Therefore, for the same reason, even terrestrial animals can be immobile.

The Philosopher says the opposite.

One must reply that an aquatic animal can be immobile more than a terrestrial one for two reasons. One is that every animal requires nourishment. Now it is certain that water is in continual movement and flux. So what now is absent from the water can be made present through the water's motion. And this is why some animals, which live on the dregs of the sea, do not need to move to seek a food source, because it is adequately carried to them by the water's motion. But this is not the case for the ground, since it is immobile, as it were, and this is why, if there were some animal fixed on the ground, it could not be preserved very long.

1. With regard to the arguments: To the first, one must say that although water may have more of form than earth, nevertheless certain aquatic animals have more crude earthiness, and so they are born to have less in the way of motion and activity. And the cause of this has already been stated.

2. To the second, one must reply that animals, unlike plants, are not born to be nourished by such an earthy food source, and so plants can continually receive their nourishment from the same place or piece of earth without motion, but animals can not. This, then, is the second cause of immobility for aquatic animals, that life depends on heat and moisture and not the dryness that is in the earth. And because earth has more dryness and water more moisture, an aquatic animal can live longer in the same place than a terrestrial animal.

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