Good grief, I’ve been remiss. Time to get back in harness.
In the second paragraph of the chapter 3 of the Isagoge, Porphyry expands on the notion of difference. I think I’m going to have to build some outlines, here.
According then, to the differences which produce another thing do the divisions of genera into species arise, and the definitions arising from genus and such differences are assigned. On the other hand, as to those which only make a thing different in quality, diversities alone consist, and the changes of subsistence of a thing; beginning then, again, from the first, we must say that of differences some are separable, others inseparable, thus to be moved, and to be at rest, to be ill, and to be well, and such as resemble these, are separable, but to have a crooked, or a flat nose, to be rational, or irrational, are inseparable differences.
Thus far, we have the following:
- In Kind vs. In Quality
- Separable vs. Inseparable
I am rational; a dog is not. This is a difference in kind. This dog is white; that dog is black. This is a difference in quality.
This dog is a Golden Retriever, but that dog is not. This is an inseparable difference. This dog is currently asleep; that dog is currently chasing a cat. This is a separable difference.
It’s clear that “inseparable” doesn’t mean “unchanging”. Being human, my nose has a shape–at all times, my nose has a shape. Even if my nose is cut off, I am a human being who is lacking a nose. I mean to say, people would notice–it would be as plain as the nose off my face.
Again, of the inseparable, some exist per se, others by accident, for rational, mortal, to be susceptible of science, are inherent in man per se, but to have a crooked or flat nose, accidentally, and not per se. Wherefore, such as are present per se, are assumed in the definition of substance, and effect a different thing, but what are accidental are neither taken in the definition of substance, nor render a thing another, but of another quality.
Aha! I need to adjust my outline.
- In Kind
- Inseparable, per se (i.e., a property)
- In Quality
- Inseparable, by accident
I’m actually not quite sure about what I have there for differences in kind. It seems to me that when Porphyry talks about differences per se he’s talking about both the specific difference, which is what makes this thing the kind of thing it is, and about other differences–properties, as they are called–which are always there for things of this kind. But I could be mistaken. I’ll note that Rune’s Dictionary of Philosophy defines a property as follows:
In Aristotle’s logic (1) an attribute common to all members of a species and peculiar to them; (2) an attribute of the above sort not belonging to the essence of the species, but necessarily following from it.
Be all that as it may, there’s a further distinction between differences that are inseparable per se and those that inseparable accidentally:
Those too, which are per se, do not admit of the more and less, but the accidental, even if they be inseparable, admit of intention and remission, for neither is genus more and less predicated of that of which it is the genus, nor the differences of genus according to which it is divided. For these are such as complete the definition of each thing, but the essence of each is one and the same, and neither admits of intention, nor remission; to have however a crooked or a flat nose, or to be in some way coloured, admits both of intension and remission.
OK; you’re an animal or you’re not. You’re rational, or you’re not. But your nose can be more or less crooked, more or less pink. I’m puzzled by the words “intention” and “remission” in this context.
Next we have a detailed example of specific differences that relate to the genus “animal”:
Since then, there are three species of difference considered, some indeed separable, but others inseparable, again, of the inseparable, some are per se, but others accidental, moreover of differences per se, some are those according to which we divide genera into species, but others according to which the things divided become specific:–thus of all such differences per se of animal as these, animated and sensitive, rational and irrational, mortal and immortal, the difference of animated and sensitive is constitutive of the essence of animal, for animal is an animated substance, endued with sense, but the difference of mortal and immortal, and that of rational and irrational, are the divisive differences of animal, for through these we divide genera into species: yet these very differences which divide the genera are constitutive and completive of species. For animal is divided by the difference of rational and irrational, and again, by the difference of mortal and immortal; but the differences of rational and mortal are constitutive of man, but those of rational and immortal of God, those again, of mortal and irrational, of irrational animals. Thus also, since the differences of animate and inanimate, sensitive and void of sense, divide the highest substance, animate and sensitive added to substance, complete animal, but animate and deprived of sense, form plant; since then, the same differences taken in one way become constitutive, but in another divisive, they are all called specific.
So rocks differ from animals in that they are not animate. Plants are animate, but differ from animals in that they are not sensitive–that is, they have no sensitive soul, no powers of sensation. Animals are sensitive but not rational, as men are; and animals are not immortal, as the gods are.
The most interesting bit of the above example is the suggestion that the difference between men and gods is that the gods are immortal. A footnote suggests that Porphyry assumes the Stoic notion of the gods; in any event, this is a facet that St. Thomas did not perpetuate.