Zoology and Demonology
We could say that a certain parallelism exists between zoology and demonology. Even though each angelic being is completely distinct from the others, since each angelic being is unique and complete in its metaphysical form(2), nevertheless, it is possible to gather them into general groups. That is, if we imagine that in each species of mammals, there existed only one specimen: one lone deer, one lone buck, one lone horse, etc., each one would be distinct, but in the zoological world we could still gather these unique beings into one general species: that of the mammals; not because they are the identical among themselves, but because they are more mutually similar than in comparison with members of the other species of insects or fishes, etc. These mammals would be mutually distinct, but than can be grouped together because there is a greater similarity amongst themselves than with the rest of living beings. The same can be said of the angelical natures. Each is distinct but can be gathered into general groups. The Bible mentions nine of them.
If the diferences between animals are often so great, in the angelical world they are even greater, because the form is freed from the laws of biology and matter. Therefore, if there is a big difference between a dragonfly and an eagle, the difference between angelical natures is much greater still. If the difference between a ladybug and a blue whale is so great, indescribably greater is the difference between an angel of the ninth hierarchy and an angel of the first.
2 Here the word "form" is used in its philosophical sense, which is distinct from the usual meaning that people give to this word. When we say that an angel is complete in its form, what we mean is the following: between human beings, for example, the "form" is the same (human), but the principle which distinguishes each individual human being is called "matter", permitting many individuals with the same "form". Since angels don't have matter, each angel must have a distinct "form" in order to distinguish itself from another angel. This is true for all beings that exist without matter. For that same reason, God has to be one and there could never be two. The divine form of Infinite Being does not have matter which permits multiplicity. Therefore, if there were two divine forms, ¿what would distinguish them? They would be one lone being; it cannot be any other way.