Isagoge: Chapter 3:3 — Of Difference

Next comes a small but confusing paragraph. We’ll take it line by line. Bear in mind that Porphyry has just finished an extended example of specific differences:

These indeed are especially useful for divisions of genera, and for definitions, yet not with regard to those which are inseparable accidentally, nor still more with such as are separable.

In other words, specific differences are useful for definitions, but not the other kinds of differences: those at are separable, and those that inseperable but accidental. As a footnote in the translation says, “accidental definition is, in fact, merely a description”. That is to say, I can describe a dog, or a breed of dog, in great detail but in doing so I haven’t captured the essence of a dog–I haven’t defined what it is to be a dog.

And indeed defining these, they say that difference is that by which species exceeds genus, e. g. man exceeds animal in being rational and mortal,

And again, this is reasonable, bearing in mind that for St. Thomas man is merely a rational animal, for all animals are mortal.

It’s the next bit that confuses me:

for animal is neither any one of these, (since whence would species have differences?) nor has it all the opposite differences, (since otherwise the same thing would at the same time have opposites,) but (as they allege) it contains all the differences which are under it in capacity, but not one of them in energy, and so neither is any thing produced from non-entities, nor will opposites at the same time subsist about the same thing.

I think what’s going on here is this: Porphyry is saying that a species has to have everything that pertains to its genus, and nothing that is opposite to its genus. I suspect that by “capacity” and “energy” Porphyry means something like “potency” and “act” (or possible vice-versa), but I’m not sure. He’s using terminology I’ve not run into before.

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