and yet the question may be raised why it is that, if indeed the female possesses the same soul and if it is the secretion of the female which is the material of the embryo, she needs the male besides instead of generating entirely from herself. the reason is that the animal differs from the plant by having sense-perception; if the sensitive soul is not present, either actually or potentially, and either with or without qualification, it is impossible for face, hand, flesh, or any other part to exist; it will be no better than a corpse or part of a corpse. if then, when the sexes are separated, it is the male that has the power of making the sensitive soul, it is impossible for the female to generate an animal from itself alone, for the process in question was seen to involve the male quality. certainly that there is a good deal in the difficulty stated is plain in the case of the birds that lay wind-eggs, showing that the female can generate up to a certain point unaided. but this still involves a difficulty; in what way are we to say that their eggs live? it neither possible that they should live in the same way as fertile eggs (for then they would produce a chick actually alive), nor yet can they be called eggs only in the sense in which an egg of wood or stone is so called, for the fact that these eggs go bad shows that they previously participate in some way in life. it is plain, then, that they have some soul potentially. what sort of soul will this be? it must be the lowest surely, and this is the nutritive, for this exists in all animals and plants alike. why then does it not perfect the parts and the animal? because they must have a sensitive soul, for the parts of animals are not like those of a plant. and so the female animal needs the help of the male, for in these animals we are speaking of the male is separate. this is exactly what we find, for the wind-eggs become fertile if the male tread the female in a certain space of time. about the cause of these things, however, we shall enter into detail later.

if there is any kind of animal which is female and has no male separate from it, it is possible that this may generate a young one from itself without copulation. no instance of this worthy of credit has been observed up to the present at any rate, but one case in the class of fishes makes us hesitate. no male of the so-called erythrinus has ever yet been seen, but females, and specimens full of roe, have been seen. of this, however, we have as yet no proof worthy of credit. again, some members of the class of fishes are neither male nor female, as eels and a kind of mullets found in stagnant waters. but whenever the sexes are separate the female cannot generate perfectly by herself alone, for then the male would exist in vain, and nature makes nothing in vain. hence in such animals the male always perfects the work of generation, for he imparts the sensitive soul, either by means of the semen or without it. now the parts of the embryo already exist potentially in the material, and so when once the principle of movement has been imparted to them they develop in a chain one after another, as the wheels are moved one by another in the automatic machines. when some of the natural philosophers say that like is brought to like, this must be understood, not in the sense that the parts are moved as changing place, but that they stay where they are and the movement is a change of quality (such as softness, hardness, colour, and the other differences of the homogeneous parts); thus they become in actuality what they previously were in potentiality. and what comes into being first is the first principle; this is the heart in the sanguinea and its analogue in the rest, as has been often said already. this is plain not only to the senses (that it is first to come into being), but also in view of its end; for life fails in the heart last of all, and it happens in all cases that what comes into being last fails first, and the first last, nature running a double course, so to say, and turning back to the point from whence she started. for the process of becoming is from the non-existent to the existent, and that of perishing is back again from the existent to the non-existent.