But neither is that which is denoted by the name fœtus an animal. But that is incorporeal, and may be called a thing and a notion, and everything rather than an animal. The nature of an animal is different. For it was clearly shown respecting the very point in question, I mean the nature of the embryo, of what sort it is. The question respecting the meanings expressed by the name animal is different.
I say, then, if you affirm that an animal is what has the power of sensation and of moving itself from appetency, that an animal is not simply what moves through appetency and is possessed of sensation. For it is also capable of sleeping, or, when the objects of sensation are not present, of not exercising the power of sensation. But the natural power of appetency or of sensation is the mark of an animal. For something of this nature is indicated by these things. First, if the fœtus is not capable of sensation or motion from appetency; which is the point proposed for consideration. Another point is; if the fœtus is capable of ever exercising the power of sensation or moving through appetency. In which sense no one makes it a question, since it is evident.
But the question was, whether the embryo is already an animal, or still a plant. And then the name animal was reduced to definition, for the sake of perspicuity. But having discovered that it is distincruished from what is not an animal by sensation and motion from appetency; we again separated this from its adjuncts; asserting that it was one thing for that to be such potentially, which is not yet possessed of the power of sensation and motion, but will some time be so, and another thing to be already so actually; and in the case of such, it is one thing to exert its powers, another to be able to exert them, but to be at rest or asleep. And this is the question.
For the embryo is not to be called an animal from the fact that it is nourished; which is the allegation of those who turn aside from the essence of the question, and apply their minds to what happens otherwise. But in the case of all conclusions alleged to be found out, demonstration is applied
