According to Aristotle the seminal liquor is secreted from the blood; and the menstrua, in females, is a similar secretion, and the only one which serves for the purpose of generation. Females, he says, have no other prolific liquor; there is, therefore, no mixture of that of the male with that of the female. He pretends to prove this from some women conceiving without receiving the least pleasure, and because few women emit this liquor externally during copulation; that in general those who are brown, and have a masculine appearance, do not emit at all, yet engender equally with those who are more fair in complexion and feminine in appearance, and whose emissions are considerable. Thus he concludes woman furnishes nothing but the menstrual. This blood is the matter of generation; and the seminal fluid of the male does not contribute as matter but as form; it is the efficient cause, the principle of motion; it is to generation what the sculptor is to a block of marble: the liquor of the male is the sculptor, the menstrual blood the marble, and the foetus the image. The menstrual blood receives from the male seed a kind of soul, which gives life and motion. This soul is neither material nor immaterial, because it can neither act upon matter nor enter in generation as matter, the menstrual blood being all that is necessary for that purpose. It is, says our philosopher, a spirit, whose substance is like that of the starry region. The heart is the first work of this soul; it contains in itself the principle of its own growth; and it has the power to arrange the other members. The menstrual blood contains every other principle of all the parts of the foetus: the soul, or spirit, of the male seed, makes the heart begin to act, and that communicates the power of bringing the other viscera to action; and thus, successively, is every part of the animal unfolded and brought into motion. All this appeared very clear to our philosopher; there only remained to him one doubt, which was, whether the heart was realized before the blood; and in fact he had reason for this doubt; for, although he had adopted the opinion of the heart existing first, Harvey has since pretended, by reasons of the same kind as those used by Aristotle, that it was not the heart but the blood which is first realized.