Isagoge: Chapter 3:4 — Of Difference

In this paragraph, Porphyry wraps up his discussion on the nature of logical difference. I’m not sure it adds much, especially given Thomas’ discussion in De Ente et Essentia, but there are a few interesting points:

Again, they define it (difference) also thus: difference is that which is predicated of many things differing in species in answer to the question, of what kind a thing is, for rational and mortal being predicated of man, are spoken in reply to what kind of thing man is, and not as to the question what is he. For when we are asked what is man, we properly answer, an animal, but when men inquire what kind of animal, we say properly, that he is rational and mortal.

Porphyry made this distinction between “what a thing is” and “what kind of a thing it is” back at the beginning of the Isagoge. Let’s expand on this a bit.

If I point at Socrates, and ask “What is that?” no one is going to answer, “Oh, that’s an animal”. They are going to say, “Oh, that’s Socrates.” If I then ask, “What is Socrates?” I might get the answer “A man.” If I ask, “What kind of a man?” I might get a variety of answers, of which “A philosopher” is probably the best. It’s when I ask “What is man?” that the answer “an animal” becomes appropriate. In other words, the question “What is X?” has a different answer depending on whether X is an individual or a species/genus. In the former case, “X” is a member of its species; in the latter, a member of its genus. And the question “What kind of Y is X?” again has a different answer depending on whether X is an individual or a species/genus.

Regarding the word “mortal”, remember that for Porphyry “mortal” is the difference that distinguishes man from the gods.

For since things consist of matter and form, or have a constitution analogous to matter and form, as a statue is composed of brass, matter, but of figure, form, so also man, both common and specific, consists of matter analogous to genus, and of form analogous to difference, but the whole of this, animal, rational, mortal, is man, in the same manner as the statue there.

Matter is analogous to genus and form analogous to difference. Hmmm. I cannot recall, at the moment, whether Thomas says that form derives from difference or from species; and the word “analogous” here strikes me as somewhat loosey-goosey. I don’t think Porphyry’s using the word quite as Thomas does.

They also describe it thus, difference is what is naturally adapted to separate things which are under the same genus, as rational and irrational separate man and horse, which are under the same genus, animal. Again, they give it in this way: difference is that by which each singular thing differs, for man and horse do not differ as to genus, for both we and horses are animals, but the addition of rational separates us from them; again, both we and the gods are rational, but the addition of mortal separates us from them. They however who more nicely discuss what pertains to difference, say that it is not any casual thing dividing those under the same genus, but such as contributes to the essence, and to the definition of the essence of a thing, and which is part of the thing. For to be naturally adapted to sail is not the difference, though it is the property of man, since we may say that of animals, some are naturally adapted to sail, but others not, separating man from other animals; yet a natural ability to sail does not complete the essence, neither is a part of it, but only an aptitude of it, because it is not such a difference as those which are called specific differences. Wherefore specific differences will be such as produce another species, and which are assumed in explaining the very nature of a thing: and concerning difference this is sufficient.

Of the preceding I found the highlighted sentence to be the most interesting. I’ve often pondered what you do with characteristics that are shared by some but not all animals. Flying, for example, is characteristic of birds, but also of insects; but not of all birds or all insects. A bird, therefore is not a “flying animal”: “flying” is not the specific difference of the species “bird”. Rather, it’s a property of some kinds of bird, and of some kinds of insect.

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