Whether every animal has every sense.
"The disposition of the instruments," etc. First one asks whether every animal has every sense.
1. It seems so. The human is like plants in the vegetative power, and like brute beasts in the sensitive power. But every plant participates in all the parts and potencies of the vegetative power with the human. Therefore, by the same argument, every brute beast participates in all the parts of the sensitive power with him.
2. Moreover, the whole intellective power is present to each one participating in it, unless it is impeded accidentally. Therefore, for the same reason, each part of the sensitive power is present to one participating in it unless it is impeded accidentally.
The Philosopher says the opposite in the third book of On the Soul and argues from reason that hearing, sight, and smell are situated in the head; but not every animal has a head, as is clear in the same chapter. Therefore, etc.
To this, one must reply that not every animal has all the senses. And the reason is this: Touch is present in an animal in order to protect it from injurious things outside itself and so that it might cleave to things that are suited to it. Taste is present, moreover, so that it may distinguish between nourishment that is proper for it and nourishment that does not suit it, when it is nearby. Smell is given to it for the sake of aliment that is somewhat distant from it, so that it will seek out that which is suitable and avoid that which is harmful. And in a similar way sight and hearing are given to animals so that they will seek out and distinguish suitable things from harmful things, when they are at a distance. Now, however, there are some animals that are immobile, and these do not have organs for progressive motion, and they therefore do not need sight or hearing or smell because if, by some of these senses, they did perceive something at a distance that is suitable for them, still they would be unable to seek it out or, if they should perceive something unsuitable or harmful, to avoid it. And this is why these three senses are present only in perfect animals having the power of progressive motion but not in those that are immobile, because these do not seek their aliment from a distance but rather it is carried to them through the water, specifically to those clinging to the sea shores, and this is why all immobile animals are aquatic and not land animals.
1. On to the arguments. To the first, one must reply that the vegetative power is not as perfect as the sensitive, and this is why it demands less diversity in the subject. On account of this in every part there can be a part of the threefold vegetative power: to wit, the nutritive, the augmentative, or the generative. But the sensitive power is more perfect and demands greater diversity in the subject, because every part requires a different organ, and this is why, etc.
Or, in another way one can say that not every plant has all the parts of the vegetative power, because some do not generate others like themselves, but are only generated from the earth, etc.
2. To the second argument one must say that intellect is immaterial and thus does not use an organ. But the sensitive power is organic and uses an organ. And this is why the intellective power is not diversified as the sensitive power is among the several and diverse organs which it uses, etc.
Whether fish have hearing.
Further one inquires whether fish have hearing.
1. It seems not. Sound is the object of hearing. But sound is not a motive force for hearing unless air that is continuous all the way to the organ of hearing acts as a medium. Since air is not continuous for fish owing to the medium of water, fish cannot hear.
2. Moreover, all things that are mute are deaf, as is said in the text. But fish are mute, therefore deaf.
The Philosopher proves the opposite through many experiments.
One must reply that fish have hearing, which the Philosopher sufficiently proves by various experiments. But their hearing is not as perfect as that of animals that live in air. The reason for this is that sound is multiplied by the mediating motion of that in which it is generated. Therefore, the most suitable medium for sound is one that is more mobile, but according to the Philosopher in the second book of On Meteorology, and according to the Commentator in book three of On Heaven and Earth, air is just this sort of medium. This is why air is a more suitable medium for sound than water, although sound can be made in both. Thus, fish do not perceive sounds that are as faint as do animals that hear in air.
1. On to the arguments. To the first, one must say that sound is caused in air and moves the continuous air as far as the hearing either through itself or through another. Now if the air extends continuously as far as the ear, then the species of the sound is multiplied in each part of the air. If the air, however, ends at the water, a certain tremor is caused by a tremor of the air that is touching the water, and the water trembles continuously as far as the hearing of the fish living in the water, and in such a case the sound is moving the continuous air through the water as far as the ear.
2. To the second argument one must say that not all things that are mute are deaf, but rather the contrary. Now we acquire variations of speech by being taught to do so. Thus a person who is deaf from birth cannot be instructed in any language [idioma], and this is why he can speak no language in an ordered way. But there is nothing that prevents there being some impediment in the nerves of the tongue while the nerves of the hearing remain well disposed.
Thus one must say with regard to the Philosopher's statement that he does not understand that all who are mute are deaf, but rather he understands that those who are mute because of deafness do not have the power of speech, etc.