We have, then, previously spoken elsewhere of both the body as a whole and its parts, explaining what each part is and for what reason it exists.But (1) the male and female are distinguished by a certain capacity and incapacity.(For the male is that which can concoct the blood into semen and which can form and secrete and discharge a semen carrying with it the principle of form- by 'principle' I do not mean a material principle out of which comes into being an offspring resembling the parent, but I mean the first moving cause, whether it have power to act as such in the thing itself or in something else- but the female is that which receives semen, indeed, but cannot form it for itself or secrete or discharge it.)And (2) all concoction works by means of heat.Therefore the males of animals must needs be hotter than the females.For it is by reason of cold and incapacity that the female is more abundant in blood in certain parts of her anatomy, and this abundance is an evidence of the exact opposite of what some suppose, thinking that the female is hotter than the male for this reason, i.e.the discharge of the catamenia.It is true that blood is hot, and that which has more of it is hotter than that which has less.But they assume that this discharge occurs through excess of blood and of heat, as if it could be taken for granted that all blood is equally blood if only it be liquid and sanguineous in colour, and as if it might not become less in quantity but purer in quality in those who assimilate nourishment properly.In fact they look upon this residual discharge in the same light as that of the intestines, when they think that a greater amount of it is a sign of a hotter nature, whereas the truth is just the opposite.For consider the production of fruit; the nutriment in its first stage is abundant, but the useful product derived from it is small, indeed the final result is nothing at all compared to the quantity in the first stage.So is it with the body;the various parts receive and work up the nutriment, from the whole of which the final result is quite small.This is blood in some animals, in some its analogue.Now since (1) the one sex is able and the other is unable to reduce the residual secretion to a pure form, and (2) every capacity or power in an organism has a certain corresponding organ, whether the faculty produces the desired results in a lower degree or in a higher degree, and the two sexes correspond in this manner (the terms 'able' and 'unable' being used in more senses than one)- therefore it is necessary that both female and male should have organs.Accordingly the one has the uterus, the other the male organs.
Again, Nature gives both the faculty and the organ to each individual at the same time, for it is better so.Hence each region comes into being along with the secretions and the faculties, as e.g.the faculty of sight is not perfected without the eye, nor the eye without the faculty of sight; and so too the intestine and bladder come into being along with the faculty of forming the excreta.And since that from which an organ comes into being and that by which it is increased are the same (i.e.the nutriment), each of the parts will be made out of such a material and such residual matter as it is able to receive.In the second place, again, it is formed, as we say, in a certain sense, out of its opposite.Thirdly, we must understand besides this that, if it is true that when a thing perishes it becomes the opposite of what it was, it is necessary also that what is not under the sway of that which made it must change into its opposite.After these premisses it will perhaps be now clearer for what reason one embryo becomes female and another male.For when the first principle does not bear sway and cannot concoct the nourishment through lack of heat nor bring it into its proper form, but is defeated in this respect, then must needs the material which it works on change into its opposite.Now the female is opposite to the male, and that in so far as the one is female and the other male.
And since it differs in its faculty, its organ also is different, so that the embryo changes into this state.And as one part of first-rate importance changes, the whole system of the animal differs greatly in form along with it.This may be seen in the case of eunuchs, who, though mutilated in one part alone, depart so much from their original appearance and approximate closely to the female form.The reason of this is that some of the parts are principles, and when a principle is moved or affected needs must many of the parts that go along with it change with it.
If then (1) the male quality or essence is a principle and a cause, and (2) the male is such in virtue of a certain capacity and the female is such in virtue of an incapacity, and (3) the essence or definition of the capacity and of the incapacity is ability or inability to concoct the nourishment in its ultimate stage, this being called blood in the sanguinea and the analogue of blood in the other animals, and (4) the cause of this capacity is in the first principle and in the part which contains the principle of natural heat- therefore a heart must be formed in the sanguinea (and the resulting animal will be either male or female), and in the other kinds which possess the sexes must be formed that which is analogous to the heart.
This, then, is the first principle and cause of male and female, and this is the part of the body in which it resides.But the animal becomes definitely female or male by the time when it possesses also the parts by which the female differs from the male, for it is not in virtue of any part you please that it is male or female, any more than it is able to see or hear by possessing any part you please.
To recapitulate, we say that the semen, which is the foundation of the embryo, is the ultimate secretion of the nutriment.By ultimate I mean that which is carried to every part of the body, and this is also the reason why the offspring is like the parent.For it makes no difference whether we say that the semen comes from all the parts or goes to all of them, but the latter is the better.But the semen of the male differs from the corresponding secretion of the female in that it contains a principle within itself of such a kind as to set up movements also in the embryo and to concoct thoroughly the ultimate nourishment, whereas the secretion of the female contains material alone.If, then, the male element prevails it draws the female element into itself, but if it is prevailed over it changes into the opposite or is destroyed.But the female is opposite to the male, and is female because of its inability to concoct and of the coldness of the sanguineous nutriment.And Nature assigns to each of the secretions the part fitted to receive it.But the semen is a secretion, and this in the hotter animals with blood, i.e.the males, is moderate in quantity, wherefore the recipient parts of this secretion in males are only passages.But the females, owing to inability to concoct, have a great quantity of blood, for it cannot be worked up into semen.Therefore they must also have a part to receive this, and this part must be unlike the passages of the male and of a considerable size.This is why the uterus is of such a nature, this being the part by which the female differs from the male.