Commentary Aristotle Physics
944. Then, at what, then, is the reason (249a13), he investigates the reason that rectilinear motions cannot be compared with circular ones. For since he had concluded that, if they are equal, then the magnitudes are equal—which seems to be unsuitable—someone might wonder whether this inability to be compared is due to the motion or to the magnitude. And this is his question: is the reason that a straight motion is not as swift as a circular one the fact that change of place is a genus containing diverse species under it (for it was said above that things of diverse species are not comparable), or is it that line is a genus containing under it straight and circular as species? Of course, time cannot be the reason, for all time is atomic—that is, indivisible—as to species.
To this question, therefore, he responds that both reasons hold, for in both cases is found a difference of species, but in such a way, nevertheless, that the diversity of species in local motion is due to the diversity of species of the magnitudes over which the motion takes place. And this is what he says: if that upon which the motion occurs has species, it follows that the local motion will have species.
945. Then, at it is also similarly (249a17), he rejects a factor that might seem to be required for identity of species and comparability in local motions. He says that changes of place are sometimes diversified by reason of the instrument—that is, by reason of that through which a local motion takes place as through an instrument; for example, if the feet are the instruments of local motion, it is walking, but if wings are, it is called flying. Yet this does not make for diversity of species in local motions, but for a diversity of figure in local motions; that is, such a diversity of motions is not according to species, but only according to a certain figure of motion, as the Commentator says.
However, it could possibly be better said that Aristotle here intends to say that changes of place are not diversified by reason of the instruments of motion, but by reason of the figure of the magnitude traversed. For it is in this way that straight and circular differ. The reason is that motions are not diversified on account of the mobiles, but on account of the things in respect to which the mobiles are moved. Now, instruments hold themselves on the part of the mobiles, while figures are on the part of that in which the motion occurs.
946. Then, at we may therefore say (249a19), he concludes his proposition.
Concerning this, he does three things:
first, he argues for the main proposition;
second, he draws from the conclusion a fact to be considered, at and this discussion serves (249a21; [947]);
third, he investigates the problem of diversity of species, at when, then, is there a difference (249a25; [948]).
He concludes first (249a19) that, since motions are not comparable unless they are of the same species, and local motions are not of one species unless they traverse magnitude of the same species, it follows that those are equally swift that traverse the same magnitude in equal time, where same refers to what is not different in species. For it is in this way that motions, too, do not differ in species. And, therefore, the main thing to be considered in the question of the comparison of motions is their differences. For if they differ either in genus or in species, they cannot be compared. But if they differ in accidents, they can be.
947. Then, at and this discussion serves (249a21), he draws from the foregoing a fact worthy of consideration: a genus is not something absolutely one, whereas a species is.
This is made known first of all from the preceding argument in which it was shown that things not of one genus are not comparable, whereas things of one species are;
second, from what was said above, in which it was stated that the nature of comparable things is one.
From this, it can be gathered that a genus is not one nature, while a species is.
The reason for this is that the species is taken from the last form, which is absolutely one in the nature of things, whereas the genus is not taken from a form that is one in the universe of things, but from one that is so in account only. For the form by which man is animal is not other than that by which man is man. Therefore, all men who are of one species agree in the form that constitutes their species, because each has a rational soul. But there is not some common soul that makes man, horse, and ass all to be animal over and above the soul that makes one a man, a horse, or an ass. (If there were, then the genus would be one and comparable, just as the species.) But it is only in our mental consideration that a generic form is extracted, namely, it is brought about by the intellect’s abstracting from the differences.
Consequently, a species is one quiddity deriving from a unity of form existing in the universe of things. The genus, however, is not one, for according to the diverse forms existing in the nature of things, diverse species are capable of receiving a same genus as a predicate. Consequently, a genus is one thing logically but not physically.
In a certain sense, a genus is one, but not purely. Rather, genera hide many joined things; that is, the equivocation of many things is often masked on account of their likeness and their closeness to a unity of genus.
Now, certain equivocal things are very unlike and possess in common only a name, as when the dog star and the animal that barks are called “dogs.” Other things, however, have a certain likeness, as when the word “man” is applied to a real man and to one that is in a painting on account of the latter’s likeness to a real man.
Yet other equivocations are very close. This may be on account of agreement in genus. For example, when “body” is said of a heavenly body and of a corruptible body, it is equivocation, because, naturally speaking, the matter is not one. They agree, however, in logical genus, for which reason they appear not to be equivocal. Or it may be on account of some likeness. For example, one who teaches in the schools is called “master” and so is the head of a house, equivocally; this is by a close equivocation, however, on account of the likeness, for each is a ruler—one in the school and the other in the house. Hence, on account of their close resemblance in genus or likeness, some things do not appear to be equivocal that nevertheless are.
948. Then, at when, then, is there a difference (249a25), he now inquires how specific differences may be taken in motions and in other things as well, because he had said that we must consider the question of the differences of motion—that is, whether motions differ specifically.
And, because the definition designates the essence of the species, he poses two questions:
one about the species;
and one about the definition.
He first of all asks about species. When is something to be reckoned of a different species from another? Is it only because the same nature is found in different receivers, as Plato held? According to the foregoing, this cannot be the case. For it has been said that a genus is not absolutely one; therefore, a difference of species is not reckoned on the basis that something is the same in two different things, except by the Platonists, who posited that a genus is absolutely one. On this account, as though answering the question, Aristotle adds specifically different not because the same thing is in a different receiver, but because a different nature is in a different receiver.
The second question is about definition. What is a term—that is, what is the definition that declares a species? And because things that have the same definition are absolutely the same, he then, as if answering the question, adds that the proper definition of a thing is that by which we can discern whether something is the same or other, for instance, white or sweet.
And “other” may be taken in two ways, as before:
in one way, as meaning that the white is said to be something other than the sweet, for in the white thing is found a subject-nature other than the one in the sweet;
in another way, as meaning that they differ not only in subject-nature, but that they are wholly not the same.