Hence accidental names said concretely are not placed in a predicament as species or genera, such as 'white' or 'musical,' except by reduction, but only according as they are signified in the abstract, such as 'whiteness' and 'music.' And because accidents are not composed of matter and form, therefore in them the genus cannot be taken from matter and the difference from form, as in composite substances; but the first genus must be taken from the very mode of being, according as being is predicated in diverse ways, according to prior and posterior, of the ten genera; just as quantity is named from the fact that it is the measure of substance, and quality according as it is a disposition of substance, and so of the others, according to the Philosopher in Book IX of the Metaphysics.
But the differences in them are taken from the diversity of the principles by which they are caused. And because the proper passions are caused by the proper principles of the subject, therefore the subject is placed in their definition in the place of a difference, if they are defined in the abstract according as they are properly in a genus, just as it is said that snubness is the curvature of a nose. But it would be the reverse if their definition were taken according as they are said concretely. For thus the subject would be placed in their definition as a genus, because then they would be defined after the manner of composite substances, in which the account of the genus is taken from matter, just as we say that the snub is a curved nose.
It is likewise also if one accident is the principle of another accident, just as the principle of relation is action and passion and quantity; and therefore the Philosopher divides relation according to these in Book V of the Metaphysics. But because the proper principles of accidents are not always manifest, therefore sometimes we take the differences of accidents from their effects, just as 'gathering together' and 'scattering apart' are called differences of color, which are caused by the abundance or scarcity of light, from which diverse species of colors are caused. CONCLUSION:
Thus, therefore, it is clear how essence exists in substances and in accidents, and how in composite and in simple substances, and in what way in all these the universal logical intentions are found—except in the First, which is at the limit of simplicity, to which the account of genus or species does not belong, and consequently neither does definition, on account of its simplicity. In Whom may there be the end and consummation of this discourse. Amen.