Whether sterility has its root in the soul or the body.

Next one inquires whether sterility has its root in the soul or the body.

1. And it seems have its root in the soul. For the generative power is a power of the soul. But sterility results from a defect in the generative power. Therefore, it comes from a defect that has its root in the soul.

2. In addition, error and falsehood can come from all things except the first [thing] as is maintained in the ninth book of the Metaphysics and the eleventh of this book. Therefore, the defect, which is sterility, has its root in the soul.

3. In addition, sterility stems from a lack of heat, spirit, and things of this sort which contribute to generation. But the soul is the principle of all of these. Therefore, it has its root in the soul.

On the contrary. Privation has its root in matter, as is said in the sixteenth book of this work. Sterility is a certain privation. Therefore, it has its root in the body.

One must respond that sterility is of two types. One is moral, and the other is natural. The moral one is found among those having discretion, as when a person wills abstinence. Thus sterility of this type has its root in the soul. But natural sterility does not have its root in the soul per se, since the powers of the soul are not weakened per se because, according to the Philosopher in the first book of On the Soul, "if an old person were to receive a youth's eye, he would see just as a youth does." Thus the visual power of the old man is not lacking in itself, but only owing to its union with an organ. And in the same way a sense is not corrupted per se by an excessive sensible, but rather owing to loss of proportion in the organ.

Thus, with respect to these arguments, one must understand that three things must be considered in terms of the soul: namely, the soul's own power; its connection with the body; and its operation through organs. Now a defect does not occur in the power itself nor, as a result, does sterility, but there can nevertheless be a defect in the soul with respect to the body and the organs.

1-3. As a result, responses are clear to the arguments, proving that sterility has its root in the soul. For the responses proceed from the body and the organs, and this per accidens, and this is why, etc.

An argument proceeds against the objection, because [sterility] is not from the soul in itself or through itself.

Whether generation of a mule from a mare and an ass is natural.

Further one inquires whether generation of a mule from a mare and an ass is natural.

1. And it seems not. Because natural generation is of like from like. But this type of generation is not of this sort. Therefore, etc.

2. In addition, natural generation comes to an end at something perfected. But this generation comes to an end at something imperfect; therefore, etc. Here is the proof of the assumption: that something is imperfect that departs from the customary operation of nature. But a mule departs from the customary operation of the generative power, and therefore, etc.

3. In addition, if this generation were natural, species would be capable of mixing together; but the conclusion is false, because species are, according to Porphyry, unmixed.

The opposite is apparent to the senses. And one can prove by reason that unnatural generation is monstrous. But this generation is not monstrous; therefore, etc.

One must say that the mule's generation can be compared to another one generated per se, and thus it is unnatural and flawed because the horse's semen is in potency to a perfected animal, and the mule is from the genus of imperfect animals. The same is true for the ass's sperm. Nevertheless, if the mule's generation is considered as a mixture of the ass's sperm with the horse's semen, this is natural, because, just as a middle color is naturally produced from a mixture of white and black, so too from the semen or menses of the mare, which are exceptionally hot, and from the sperm of the ass, which is exceptionally cold, a middle one can naturally come to be departing from each of the extremes. Thus, when the ass's sperm acts on the mare's semen, it does not entirely prevail over the matter, nor it is entirely overcome by it. And this is why, although it cannot produce one entirely like itself, it does, so far as it can, produce one similar to itself. With respect to the fetus's shape, it resembles its mother more, because size is consequent upon matter. But with respect to its complexion, it resembles the father.

1, 3. With this a response to the arguments is clear. For they prove that this generation is unnatural, and this is true when considering it in relation to each of the ones generating per se.

2. To the second argument one must reply that although the mule belongs to the genus of imperfect animals, its generation is nevertheless natural in a certain respect because there are many animals that are imperfect when compared to others whose generation is natural because they are perfect in their own genus.

Whether the mule and she-mule are sterile by nature.

Further one inquires whether the mule and she-mule are sterile by nature.

1. It seems not. Because an offspring is generated from a dog and a wolf that is capable of generating, and so too from a hen and a partridge [cubeg], and so too this one will be capable of generating for the same reason.

2. In addition, one temperate [in complexion] is generated from one that is temperate. But the ass's matter is tempered by the mare's semen, and contrariwise. Therefore, etc.

The Philosopher says the opposite.

One must say that the mule and she-mule are naturally sterile. And the cause for this is threefold. One is the proportion of the semen, which the Philosopher touches upon, because the ass's sperm is very cold and even a little of the mare's semen is hot. Thus, when the ass's sperm is joined to the mare's menses, each withdraws from its own nature since the sperm becomes warm and the menses become cold beyond their own nature, and this is why an animal is generated that departs from nature. The second cause is a defect in moisture. For the mule and she-mule are so deficient in spermatic moisture that, if the she-mule were to conceive and become pregnant, she could not nourish the fetus [conceptus] until the time of its birth. And likewise, if the mule should produce sperm, it would be dry because the moisture of the she-mule's menses would consume a great deal of it. Another cause is the indisposition of the parts serving generation, because the mule is an animal with a harsh complexion. Thus the instrument of both the male and the female hardens past any use for generation.

1. On to the arguments. To the first, one must respond that although an animal is generated from a dog and a wolf, or a hen and a partridge, the one does not differ in species from the other, and this is owing to the suitability of the semen. But this is not so in the case proposed owing to the disagreement of the semen between the ass and the mare.

2. The other argument proves that a mule can be generated from an ass and a horse, but it does not prove that a mule generates.