by genus, or “one” by relation, or “one” by accident, is subsumed under what we call “one in the universal species,” that is, one by genus, class, or kind.
As for the eighth designation that they count under the heading of “one” in their technical usage, it is “one by subject.” An example is the statement, “The color, scent, and taste are one,” meaning that their locus and subject are one and the same—namely, the rose. This category takes in both what is one in concrete individuality and what is one in genus: the single color, scent, and taste that inhere in a single object share one and the same subject in concreto, and what is one in genus resides in a subject that is one in genus. The items they treat as both common and distinctive—such as existence and quiddity—resemble this category. Thus, when it is said, “A single nature does not entail attributes that differ by species but only those that are single in species, and a single nature in concreto does not entail attributes that differ in concreto but only those that are single in concreto,” and someone adds, “The nature of necessary existence is one, so differing quiddities cannot attach to it,” the answer is: “The nature that is one in species—the very species-nature of the Necessary Existent—does not require quiddities that differ in species, but a quiddity that is one in species. Likewise, the determinate nature of the Necessary Existent does not require multiple quiddities; rather, it requires one determinate quiddity.”
They went astray only because they observed that a nature which is one in species is not accompanied by individual quiddities that differ in concrete existence—and that observation is correct. Yet it must be said to them: a nature that is one in species is not bound to any particular instance—neither to a determinate existence, nor to a determinate quiddity, nor to anything else. The very species-nature is not tied to its individual instantiations; rather, the individuals are tied to the species once they have been individuated. One must also tell them: that which is one in species is, in the external world, determinate, and the intended quiddity is inseparable from the determinate existence, not from existence in the abstract or from the undifferentiated nature. (1)