Isagoge: Chapter 3:1 — Of Difference

Now, you know and I know that when Thomas speaks of “difference” he means the specific difference, that which distinguishes species from another within the same genus. Man is a rational animal, that is, an animal that is rational; animal is his species genus, and rational his difference.

Porphyry, in his usual way, begins with the common meaning of the word “difference,” and moves on from there.

Difference may be predicated commonly, properly, and most properly:….

So: three kinds of difference: common, proper, and most proper.

….for one thing is said to differ from another in common from its differing in some respect in diversity of nature, either from itself, or from something else; for Socrates differs from Plato in diversity of nature, and himself from himself when a boy, and when become a man, also when he does any thing, or ceases to do it, and it is always perceived in the different ways in which a thing is somehow effected.

In the common sense, this differs from that when, well, this differs from that. I think we get that.

Again, one thing is said to differ properly from another, when one differs from another by an inseparable accident; but an inseparable accident is such as blueness, or crookedness, or a scar become scirrhous from a wound.

That is, when they differ in quality.

Moreover, one is most properly said to differ from another, when it varies by specific difference, as man differs from horse by specific difference, i. e. by the quality of rational.

So specific difference is the most proper use of the term.

Universally then every difference acceding to a thing renders it different, but differences common and proper render it different in quality, and the most proper render it another thing.

Aha! A white dog and a black dog are both dogs, but two things that differ in the most proper way, by a specific difference, are two different kinds of thing. Which is, of course, why it’s called a “specific” difference:

Hence, those which render it another thing are called specific, but those, which make it different in quality, are simply (called) differences, for the difference of rational being added to animal, makes it another thing, (and makes a species of animal,) but difference of being moved makes it different in quality only from what is at rest, so that the one renders it another thing, but the other only of another quality.

So far, so good. I confess, I’m not entirely sure why Porphyry distinguishes above between common and proper differences, but I’m not sure that it matters, either.

3 Responses to “Isagoge: Chapter 3:1 — Of Difference”

  1. Peter says:

    Animal is man’s genus, not his species. The species is the genus + specific difference: rational animal.

  2. Will says:

    Whoops! Thanks for the correction.

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