Isagoge: Chapter 8 — Of Community and Difference of Genus and Species

Once again, we compare and contrast. Here’s what Porphyry has to say:

Genus and species possess in common, (as we have said,) the being predicated of many things, but species must be taken as species only, and not as genus, if the same thing be both species and genus. Moreover, it is common to them both to be prior to what they are predicated of, and to be each a certain whole; but they differ, because genus indeed comprehends species, but species are comprehended by, and do not comprehend genera, for genus is predicated of more than species. Besides, it is necessary that genera should be presupposed, and when formed by specific differences, that they should consummate species, whence also genera are by nature prior. They also co-subvert, but are not co-subverted, for species existing, genus also entirely exists, but genus existing there is not altogether species; genera too, are indeed univocally predicated of species under them, but not species of genera. Moreover, genera exceed, from comprehending the species which are under them, but species exceed genera by their proper differences; besides, neither can species become most generic, nor genus most specific.

I’m not going to go through this line by line; instead, I’m going to list the points I take away from it.

Aquinas frequently uses the word aspect, as in, “under the aspect of”. I’ve never felt very clear as to just what he means by it, but reflecting on this passage has helped bring me closer to it. An aspect, I take it, is a way of looking at something, a way of analyzing it. Most things can be looked at in different ways.

Now, genus and species have much in common, because all species but the most specific are also genera, and all genera but the most generic are also species. Take animal, for example, which is a species of body, but a genus containing a vast number of species, including Man. But while most genera are species, and most species are genera, it is not the case that genus and species are the same thing. We can consider body as a genus, and as a species, and these are two different aspects.

OK, so what else do we learn from Porphyry?

  • Genus and species are both predicated of many things.
  • They are prior to that of which they are predicated, that is, they are causes in the Aristotelian sense.
  • They are each a certain whole (I’m not at all sure what this means).
  • A genus is prior to the species within it.
  • Equivalently, a genus is predicated of the species within it, but not vice versa.
  • Given genus G and species S within it, G and S co-subvert; I gather that this means that if X is not an S, then it isn’t a G either, and if it isn’t a G, then it isn’t an S.
  • On the other hand, G and S are not co-subverted; which I gather means that if X is an S you know it’s also a G, but if you know that it’s a G you don’t know whether it’s an S or not.

The talk of genera comprehending species and species not comprehending genera bugs me. As I understand it, the comprehension of a genus or species is what it means; the extension of a genus or species is the collection of beings of which it is predicated. The comprehension of man is wider than that of animal because it takes the comprehension of animal and adds rational to it; whereas the extension of man is narrower than that of animal because all men are animals but not all animals are men.

That latter appears to be what Porphyry is saying, or in other words, that animal includes man, and other species, but man doesn’t include animal in the same way. But I don’t see that this use of “comprehend” aligns with the description I’ve just given.

2 Responses to “Isagoge: Chapter 8 — Of Community and Difference of Genus and Species”

  1. A couple of clarifications I hope will help:

    “a certain whole” means they are universals; think of universals in relation to particulars as wholes are related to their parts. He uses the word ‘certain’ to show it’s not exactly the same as with material composition, but in a certain way it is like it.

    “aspect” etymologically is “ad-spicere”, to look to, so it has to do with how something is perceived, as you say, as opposed to how it is in itself; aspect could be in some way opposed to feature, or quality. To say something “has this aspect” differs from saying it has this or that quality, something analogous to the difference between denotation (ontological/quality) vs connotation (perception/aspect).

    Finally, regarding comprehension, it more widely means being grasped, like the German “Begriff” (translated concept) (think of prehensile tail, ie a tail that grasps). Comprehension is something that totally grasps something else; eg God comprehends us, but we cannot comprehend God, we cannot get our mental fingers around him to grasp him.

  2. Will says:

    Aha! “Certain” as in “In a certain way”, rather than as in “absolutely certain”. OK; that makes sense.

    And then, he’s simply using “comprehend” in a more general way. (I love the distinction between “apprehend”, to seize, and “comprehend”, to grasp.) In this case, he seems to mean something like “includes all of”.