We can see how this happens by considering how body as a part of animal differs from body as the genus of animal. In the way body is the genus of animal it cannot be an integral part of animal, and thus the term body can be accepted in several ways. Body is said to be in the genus of substance in that it has a nature such that three dimensions can be designated in the body. These three designated dimensions are the body that is in the genus of quantity. Now, it sometimes happens that what has one perfection may attain to a further perfection as well, as is clear in man, who has a sensitive nature and, further, an intellective one. Similarly, above this perfection of having a form such that three dimensions can be designated in it, there can be joined another perfection, as life or some similar thing. This term body, therefore, can signify a certain thing that has a form such that from the form there follows in the thing designatability in three dimensions and nothing more, such that, in other words, from this form no further perfection follows, but if some other thing is superadded, it is beyond the signification of body thus understood. And understood in this way, body will be an integral and material part of the animal, because in this way the soul will be beyond what is signified by the term body, and it will supervene on the body such that from these two, namely the soul and the body, the animal is constituted as from parts.
This term body can also be understood as signifying a certain thing that has a form such that three dimensions can be designated in it, whatever form this may be, and such that either from the form some further perfection can proceed or not. Understood in this way, body will be the genus of animal, for there will be understood in animal nothing that is not implicitly contained in body. Now, the soul is a form through which there can be designated in the thing three dimensions, and therefore, when we say that body is what has a form from which three dimensions can be designated in the body, we understand there is some kind of form of this type, whether soul, or lapideousness, or whatever other form. And thus the form of animal is implicitly contained in the form of body, just as body is its genus.
The relation of animal to man is the same. For if animal named just a certain thing that has a perfection such that it can sense and move by a principle existing in itself, without any other perfection, then whatever further perfection may supervene would be related to animal as a component part, and not as implicitly contained in the notion of animal; and in this way animal would not be a genus. But animal is a genus in that it signifies a certain thing from the form of which sensation and motion can proceed, whatever this form may be, whether a sensible soul only, or a soul both sensible and rational.
Therefore, the genus signifies indeterminately the whole that is in the species and does not signify matter alone. Similarly, the difference also signifies the whole and does not signify the form alone, and the definition, or even the species, signifies the whole. But these nevertheless signify the same thing in different ways. For the genus signifies the whole as a certain denomination determining that which is material in the thing without a determination of its proper form, whence the genus is taken from the matter, although it is not the matter. This is clear in the case of bodies, as we call something a body in that the thing has a perfection such that in the thing three dimensions can be designated, and this perfection is related materially to some further perfection. Conversely, the difference is like a certain denomination taken from the determined form, beyond the first conception of the form by which the matter is determined. So, when we say something is animated (that, in other words, it has a soul), this does not determine what the thing is, whether it is a body or some other thing. Hence, Avicenna says, Metaphysicae V, cap. 6, that the genus is not understood in the difference as a part of its essence but only as a being beyond its essence, even as a subject is with respect to the concept of a passion. And thus the genus is not predicated per se of the difference, as the Philosopher says in III Metaphysicae cap. 8 (998b24) and in IV Topicorum cap. 2 (122b22-26), unless perhaps as a subject is predicated of a passion. But the definition or the species comprehends both, namely, the determined matter that the term genus designates and the determined form that the term difference designates.
