1. Attributes of the same subject representing different forms are predicated of one another accidentally: thus 'white' is said to be 'musical' accidentally, inasmuch as whiteness and music happen both to be in Socrates. If then the intelligent, sentient, and vegetative soul are different powers or forms in us, then the attributes that we have according to these forms will be predicated of one another accidentally. But according to the intelligent soul we are called 'men,' according to the sentient 'animals,' according to the vegetative 'living.' This then will be an accidental predication, 'man is an animal,' or 'an animal is a living creature.' But on the contrary these are cases of essential predication: for man, as man, is an animal; and an animal, as an animal, is a living creature. Therefore it is from the same principle that one is man, animal, and alive.*
2. A thing has unity from the same principle whence it has being, for unity is consequent upon being. Since then everything has being from its form, it will have unity also from its form. If therefore there are posited in man several souls, as so many forms, man will not be one being but several. Nor will the order of the forms to one another, one ensuing upon the other, suffice for the unity of man: for unity in point of orderly succession is not absolute unity: such unity of order in fact is the loosest of unities.*
4. If man, as Plato held, is not a compound of soul and body, but is a soul using a body; either this is understood of the intelligent soul, or of the three souls, if there are three, or of two of them. If of three, or two, it follows that man is not one, but two, or three: for he is three souls, or at least two. But if this is understood of the intelligent soul alone, so that the sentient soul is to be taken for the form of the body, and the intelligent soul, using the animate and sentient body, is to be man, there will still ensue awkward consequences, to wit, that man is not an animal, but uses an animal; and that man does not feel, but uses a thing that does feel.
5. Of two or three there cannot be made one without anything to unite them, unless one of them stands to the other as actuality to potentiality: for so of matter and form there is made one without any external bond to bind them together. But if in man there are several souls, they do not stand to one another as matter and form, but they are all supposed to be actualities and principles of action. If then they are to be united to make one man, or one animal, there must be something to unite them. This cannot be the body, since rather the body is made one by the soul: the proof of which fact is that, when the soul departs, the body breaks up. It must be some more formal principle that makes of those several entities one; and this will be rather the soul than those several entities which are united by it. If this again has several parts, and is not one in itself, there must further be something to unite those parts. As we cannot proceed to infinity, we must come to something which is in itself one; and this of all things is the soul.* There must therefore in one man, or one animal, be one only soul.
2.57 : Plato's Theory of the Union of the Intellectual Soul with the Body
2.59 : That the Potential Intellect of Man is not a Spirit subsisting apart from Matter