The Complete Aristotle (eng.)

specific differences either in the attribute itself or in that
which contains the attribute-that these, I mean, must not be
divisible in the way in which colour is divided into kinds? Thus in
this respect one thing will not be commensurable with another, i.e.
we cannot say that one is more coloured than the other where only
colour in general and not any particular colour is meant; but they
are commensurable in respect of whiteness.
    Similarly in the case of motion: two things are of the same
velocity if they occupy an equal time in accomplishing a certain
equal amount of motion. Suppose, then, that in a certain time an
alteration is undergone by one half of a body’s length and a
locomotion is accomplished the other half: can be say that in this
case the alteration is equal to the locomotion and of the same
velocity? That would be absurd, and the reason is that there are
different species of motion. And if in consequence of this we must
say that two things are of equal velocity if they accomplish
locomotion over an equal distance in an equal time, we have to
admit the equality of a straight line and a circumference. What,
then, is the reason of this? Is it that locomotion is a genus or
that line is a genus? (We may leave the time out of account, since
that is one and the same.) If the lines are specifically different,
the locomotions also differ specifically from one another: for
locomotion is specifically differentiated according to the specific
differentiation of that over which it takes place. (It is also
similarly differentiated, it would seem, accordingly as the
instrument of the locomotion is different: thus if feet are the
instrument, it is walking, if wings it is flying; but perhaps we
should rather say that this is not so, and that in this case the
differences in the locomotion are merely differences of posture in
that which is in motion.) We may say, therefore, that things are of
equal velocity in an equal time they traverse the same magnitude:
and when I call it ‘the same’ I mean that it contains no specific
difference and therefore no difference in the motion that takes
place over it. So we have now to consider how motion is
differentiated: and this discussion serves to show that the genus
is not a unity but contains a plurality latent in it and distinct
from it, and that in the case of equivocal terms sometimes the
different senses in which they are used are far removed from one
another, while sometimes there is a certain likeness between them,
and sometimes again they are nearly related either generically or
analogically, with the result that they seem not to be equivocal
though they really are.
    When, then, is there a difference of species? Is an attribute
specifically different if the subject is different while the
attribute is the same, or must the attribute itself be different as
well? And how are we to define the limits of a species? What will
enable us to decide that particular instances of whiteness or
sweetness are the same or different? Is it enough that it appears
different in one subject from what appears in another? Or must
there be no sameness at all? And further, where alteration is in
question, how is one alteration to be of equal velocity with
another? One person may be cured quickly and another slowly, and
cures may also be simultaneous: so that, recovery of health being
an alteration, we have here alterations of equal velocity, since
each alteration occupies an equal time. But what alteration? We
cannot here speak of an ‘equal’ alteration: what corresponds in the
category of quality to equality in the category of quantity is
‘likeness’. However, let us say that there is equal velocity where
the same change is accomplished in an equal time. Are we, then, to
find the commensurability in the subject of the affection or in the
affection itself? In the case that we have just been considering it
is the fact that health is one and the same that enables us to
arrive at the conclusion that the one alteration is neither more
nor less than the other, but that both are alike. If on the other
hand the affection is different in the two cases, e.g. when the
alterations take the form of becoming white and becoming healthy
respectively, here there is no sameness or equality or likeness
inasmuch as the difference in the affections at once makes the
alterations specifically different, and there is no unity of
alteration any more than there would be unity of locomotion under
like conditions.