Here we are, with the penultimate paragraph of the discourse.
The differences in accidents are taken from the diversity of principles by which they are caused.
This statement baffled me for a moment; but then I went back to the definitions of the terms.
Logically, a species is a genus and a difference. So the implied question is, what differentiates one accident from another? The “diversity of principles by which they are caused.” And remember that a “principle” is the first cause of something. That’s an important distinction between philosophy and experimental science. Experimental science is usual looking for the immediate, or proximate cause of something; philosophy is looking for the ultimate cause. When we say, (as I seem to have been saying repeatedly for months now) that “matter is the principle of individuation”, I’m saying that matter is what causes individuals to be individual.
Since passions are properly caused by the proper principles of the subject, the subject is placed in the definition of the passion in place of the difference if the passion is being defined in the abstract and properly in its genus, as when we say that having a snubnose is the upward curvature of the nose.
A property of a substance is something inessential that is nevertheless always true, i.e., something that follows necessarily from the substance’s nature. I would surmise, then, that the proper principles of a subject are the principles, the most important causes, of the subject’s properties.
The term passion confuses me; it doesn’t mean what we usually think of when we think about passion. If I understand it correctly, a passion is something that a substance does passively. Since they are “properly caused” by the “proper principles”, one imagines that passions are indeed properties of the substance, so I’ll go with that.
Thomas says, “the subject is placed in the definition of the passion in place of the difference if the passion is being defined in the abstract….” In the example, the genus is “upward curvature” and the difference, or in this case, the subject, is “the nose”. A snubnose has an upward curvature, indeed, and it always has it; this is a property, a passion, of a snubnose.
But it would be the converse if the definition of the passion were taken according to its concrete sense; in this way, the subject is placed in the definition as a genus, for then the passion is defined in the mode of composite substances in which the notion of the genus is taken from the matter, as when we say that a snubnose is an upwardly curving nose.
In the abstract sense, we’re talking about snubness of noses; in the concrete sense, we’re talking about noses that are snub. So here, the genus is nose and the difference is upwardly curving.
The case is similar when one accident is the principle of another, as the principle of relation is action and passion and quantity, and thus by reference to these the Philosopher divides relation in V Metaphysicae cap. 15 (1020b26-32).
A relation consists of, is caused by, an action, a passion, and a quantity. I’d like to hear an explanation of that.
But because the proper principles of accidents are not always manifest, we sometimes take the differences of accidents from their effects, as we do with the concentrative and the diffusive, which are called the differences of color and which are caused by the abundance or the paucity of light, which cause the different species of color.
What makes all of this so hard is that there’s so much of it that we can’t directly perceive, and things that we can know in theory we can’t know in practice. We can know that accidents have proper principles…but we can’t necessarily know what they are. And so we go with what we can know, such as the effect of the accidents.