Whether sex is necessary for the generation of animals.

We have already determined above," etc. With regard to this fifteenth book, one first inquires into sex, and whether sex is necessary for the generation of animals.

1. And it seems not. For according to the Philosopher generation exists because the individual is corruptible. Therefore, the generative power is implanted in it so that it can be preserved in species, although not in itself. Therefore, generation corresponds to corruption. But corruption can occur without any distinction of the sexes, and therefore so too can generation.

2. In addition, according to the Philosopher in the second book of On the Soul and the fourth book of the Metaphysics, each one is perfect when it can make one like itself. Therefore, since an animal is more perfect than a plant, and plants can generate one like themselves by themselves, so much more so can an animal.

3. In addition, a male is able to generate in another; a female, however, generates in herself. But these facts suit the elements as well as animated beings, for fire can generate in another and something can be generated in fire. Therefore, just as sex is not required for the generation of the elements, neither will it be required for the generation of animals.

The Philosopher says the opposite.

One must reply that generation is of two kinds. One is the transmutation of one by another, and in this type the one generating disposes the matter of the other and introduces form, and this kind of generation is attributed to the elements. The other type is generation by division or a cutting-off of a part from the whole, or of one from another, and in this type of generation not only is form introduced by the ones generating, but also the matter is provided by them. And because nature always intends what is better, and what is better is nobler, in things that are acted upon, nature separates off the more ignoble. And the agent is more noble than the one being acted upon, just as form is nobler than matter, and this is why, in the generation of animals, nature separates the male as the nobler from the female, because perfect animals generate by the separation of semen, and in this separation the male is just like the agent, and the female is just like the one being acted upon. And this is why sex is necessary for the generation of ones like these.

In addition, sperm is not always suited for generation everywhere but only in the appropriate place, a place, moreover, receptive of sperm, so that it is suitable for generation. This exists only in the female or in a part emitted by the female. And he says this about fish, because the female emits the future offspring or eggs, and the male follows behind and spreads his sperm over them. And this is why sex is necessary for the generation of animals.

1. On to the arguments. To the first, one must reply that generation proceeds by way of nature. But every corruption is in a certain sense contrary to nature, and this is why more things are required for generation than for corruption, because nature's operation proceeds in an ordered way, and an operation that is contrary to nature proceeds without order.

2. To the second argument one must reply that the more perfect something is in the lower realms, the more things are required for its generation, as is said in the second book of On Heaven and Earth? And this is why, although an animal is more perfect than a plant, nevertheless it cannot be generated from only one thing as a plant can, but rather requires several things for its generation owing to its perfection.

3. To the third argument one must reply that although an element generates in another and in itself, nevertheless this is said equivocally with respect to an animal. For the generation of fire, it is not required that there be one fire as agent and that there be another sustaining the action for the one that is generated.

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