The Complete Aristotle (eng.)

the same reasons for
which being and substance cannot be genera.
    Further, the position must be similar in all the kinds of unity.
Now ‘unity’ has just as many meanings as ‘being’; so that since in
the sphere of qualities the one is something definite-some
particular kind of thing-and similarly in the sphere of quantities,
clearly we must in every category ask what the one is, as we must
ask what the existent is, since it is not enough to say that its
nature is just to be one or existent. But in colours the one is a
colour, e.g. white, and then the other colours are observed to be
produced out of this and black, and black is the privation of
white, as darkness of light. Therefore if all existent things were
colours, existent things would have been a number, indeed, but of
what? Clearly of colours; and the ‘one’ would have been a
particular ‘one’, i.e. white. And similarly if all existing things
were tunes, they would have been a number, but a number of
quarter-tones, and their essence would not have been number; and
the one would have been something whose substance was not to be one
but to be the quarter-tone. And similarly if all existent things
had been articulate sounds, they would have been a number of
letters, and the one would have been a vowel. And if all existent
things were rectilinear figures, they would have been a number of
figures, and the one would have been the triangle. And the same
argument applies to all other classes. Since, therefore, while
there are numbers and a one both in affections and in qualities and
in quantities and in movement, in all cases the number is a number
of particular things and the one is one something, and its
substance is not just to be one, the same must be true of
substances also; for it is true of all cases alike.
    That the one, then, in every class is a definite thing, and in
no case is its nature just this, unity, is evident; but as in
colours the one-itself which we must seek is one colour, so too in
substance the one-itself is one substance. That in a sense unity
means the same as being is clear from the facts that its meanings
correspond to the categories one to one, and it is not comprised
within any category (e.g. it is comprised neither in ‘what a thing
is’ nor in quality, but is related to them just as being is); that
in ‘one man’ nothing more is predicated than in ‘man’ (just as
being is nothing apart from substance or quality or quantity); and
that to be one is just to be a particular thing.
<
    div id="section111" class="section" title="3">
3
    The one and the many are opposed in several ways, of which one
is the opposition of the one and plurality as indivisible and
divisible; for that which is either divided or divisible is called
a plurality, and that which is indivisible or not divided is called
one. Now since opposition is of four kinds, and one of these two
terms is privative in meaning, they must be contraries, and neither
contradictory nor correlative in meaning. And the one derives its
name and its explanation from its contrary, the indivisible from
the divisible, because plurality and the divisible is more
perceptible than the indivisible, so that in definition plurality
is prior to the indivisible, because of the conditions of
perception.
    To the one belong, as we indicated graphically in our
distinction of the contraries, the same and the like and the equal,
and to plurality belong the other and the unlike and the unequal.
‘The same’ has several meanings; (1) we sometimes mean ‘the same
numerically’; again, (2) we call a thing the same if it is one both
in definition and in number, e.g. you are one with yourself both in
form and in matter; and again, (3) if the definition of its primary
essence is one; e.g. equal straight lines are the same, and so are
equal and equal-angled quadrilaterals; there are many such, but in
these equality constitutes unity.
    Things are like if, not being absolutely the same, nor without
difference in respect of their concrete substance, they are the
same in form; e.g. the larger square is like the smaller, and
unequal straight lines are like; they are like, but not absolutely
the same. Other things are like, if, having the same form, and
being things in which difference of degree is possible, they have
no difference of degree. Other things, if they have a quality that
is in form one and same-e.g. whiteness-in a greater or less degree,
are called like because their form is one. Other