The Complete Aristotle (eng.)

still
higher degree; and especially if a thing is of this sort by nature,
and not by force like the things which are unified by glue or nails
or by being tied together, i.e. if it has in itself the cause of
its continuity. A thing is of this sort because its movement is one
and indivisible in place and time; so that evidently if a thing has
by nature a principle of movement that is of the first kind (i.e.
local movement) and the first in that kind (i.e. circular
movement), this is in the primary sense one extended thing. Some
things, then, are one in this way, qua continuous or whole, and the
other things that are one are those whose definition is one. Of
this sort are the things the thought of which is one, i.e. those
the thought of which is indivisible; and it is indivisible if the
thing is indivisible in kind or in number. (3) In number, then, the
individual is indivisible, and (4) in kind, that which in
intelligibility and in knowledge is indivisible, so that that which
causes substances to be one must be one in the primary sense.
‘One’, then, has all these meanings-the naturally continuous and
the whole, and the individual and the universal. And all these are
one because in some cases the movement, in others the thought or
the definition is indivisible.
    But it must be observed that the questions, what sort of things
are said to be one, and what it is to be one and what is the
definition of it, should not be assumed to be the same. ‘One’ has
all these meanings, and each of the things to which one of these
kinds of unity belongs will be one; but ‘to be one’ will sometimes
mean being one of these things, and sometimes being something else
which is even nearer to the meaning of the word ‘one’ while these
other things approximate to its application. This is also true of
‘element’ or ‘cause’, if one had both to specify the things of
which it is predicable and to render the definition of the word.
For in a sense fire is an element (and doubtless also ‘the
indefinite’ or something else of the sort is by its own nature the
element), but in a sense it is not; for it is not the same thing to
be fire and to be an element, but while as a particular thing with
a nature of its own fire is an element, the name ‘element’ means
that it has this attribute, that there is something which is made
of it as a primary constituent. And so with ‘cause’ and ‘one’ and
all such terms. For this reason, too, ‘to be one’ means ‘to be
indivisible, being essentially one means a “this” and capable of
being isolated either in place, or in form or thought’; or perhaps
‘to be whole and indivisible’; but it means especially ‘to be the
first measure of a kind’, and most strictly of quantity; for it is
from this that it has been extended to the other categories. For
measure is that by which quantity is known; and quantity qua
quantity is known either by a ‘one’ or by a number, and all number
is known by a ‘one’. Therefore all quantity qua quantity is known
by the one, and that by which quantities are primarily known is the
one itself; and so the one is the starting-point of number qua
number. And hence in the other classes too ‘measure’ means that by
which each is first known, and the measure of each is a unit-in
length, in breadth, in depth, in weight, in speed. (The words
‘weight’ and ‘speed’ are common to both contraries; for each of
them has two meanings-’weight’ means both that which has any amount
of gravity and that which has an excess of gravity, and ‘speed’
both that which has any amount of movement and that which has an
excess of movement; for even the slow has a certain speed and the
comparatively light a certain weight.)
    In all these, then, the measure and starting-point is something
one and indivisible, since even in lines we treat as indivisible
the line a foot long. For everywhere we seek as the measure
something one and indivisible; and this is that which is simple
either in quality or in quantity. Now where it is thought
impossible to take away or to add, there the measure is exact
(hence that of number is most exact; for we posit the unit as
indivisible in every respect); but in all other cases we imitate
this sort of measure. For in the case of a furlong or a talent or
of anything comparatively large any addition or subtraction might
more easily escape our notice than in the case of something
smaller; so that the first thing from which, as far as