These distinctions having been drawn, it is evident that every
change and everything that moves is in time; for the distinction of
faster and slower exists in reference to all change, since it is found
in every instance. In the phrase "moving faster" I refer to that which
changes before another into the condition in question, when it moves
over the same interval and with a regular movement; e.g. in the case
of locomotion, if both things move along the circumference of a
circle, or both along a straight line; and similarly in all other
cases. But what is before is in time; for we say "before" and
"after" with reference to the distance from the "now", and the "now"
is the boundary of the past and the future; so that since "nows" are
in time, the before and the after will be in time too; for in that
in which the "now" is, the distance from the "now" will also be. But
"before" is used contrariwise with reference to past and to future
time; for in the past we call "before" what is farther from the "now",
and "after" what is nearer, but in the future we call the nearer
"before" and the farther "after". So that since the "before" is in
time, and every movement involves a "before", evidently every change
and every movement is in time.

    It is also worth considering how time can be related to the soul;
and why time is thought to be in everything, both in earth and in
sea and in heaven. Is because it is an attribute, or state, or
movement (since it is the number of movement) and all these things are
movable (for they are all in place), and time and movement are
together, both in respect of potentiality and in respect of actuality?

    Whether if soul did not exist time would exist or not, is a question
that may fairly be asked; for if there cannot be some one to count
there cannot be anything that can be counted, so that evidently
there cannot be number; for number is either what has been, or what
can be, counted. But if nothing but soul, or in soul reason, is
qualified to count, there would not be time unless there were soul,
but only that of which time is an attribute, i.e. if movement can
exist without soul, and the before and after are attributes of
movement, and time is these qua numerable.

    One might also raise the question what sort of movement time is
the number of. Must we not say "of any kind"? For things both come
into being in time and pass away, and grow, and are altered in time,
and are moved locally; thus it is of each movement qua movement that
time is the number. And so it is simply the number of continuous
movement, not of any particular kind of it.

    But other things as well may have been moved now, and there would be
a number of each of the two movements. Is there another time, then,
and will there be two equal times at once? Surely not. For a time that
is both equal and simultaneous is one and the same time, and even
those that are not simultaneous are one in kind; for if there were
dogs, and horses, and seven of each, it would be the same number.
So, too, movements that have simultaneous limits have the same time,
yet the one may in fact be fast and the other not, and one may be
locomotion and the other alteration; still the time of the two changes
is the same if their number also is equal and simultaneous; and for
this reason, while the movements are different and separate, the
time is everywhere the same, because the number of equal and
simultaneous movements is everywhere one and the same.

    Now there is such a thing as locomotion, and in locomotion there
is included circular movement, and everything is measured by some
one thing homogeneous with it, units by a unit, horses by a horse, and
similarly times by some definite time, and, as we said, time is
measured by motion as well as motion by time (this being so because by
a motion definite in time the quantity both of the motion and of the
time is measured): if, then, what is first is the measure of
everything homogeneous with it, regular circular motion is above all
else the measure, because the number of this is the best known. Now
neither alteration nor increase nor coming into being can be
regular, but locomotion can be. This also is why time is thought to be
the movement of the sphere, viz. because the other movements are
measured by this, and time by this movement.

    This also explains the common saying that human affairs form a
circle, and that there is a circle in all other things that have a
natural movement and coming into being and passing away. This is
because all other things are discriminated by time, and end and
begin as though conforming to a cycle; for even time itself is thought
to be a circle. And this opinion again is held because time is the
measure of this kind of locomotion and is itself measured by such.
So that to say that the things that come into being form a circle is
to say that there is a circle of time; and this is to say that it is
measured by the circular movement; for apart from the measure
nothing else to be measured is observed; the whole is just a plurality
of measures.

    It is said rightly, too, that the number of the sheep and of the
dogs is the same number if the two numbers are equal, but not the same
decad or the same ten; just as the equilateral and the scalene are not
the same triangle, yet they are the same figure, because they are both
triangles. For things are called the same so-and-so if they do not
differ by a differentia of that thing, but not if they do; e.g.
triangle differs from triangle by a differentia of triangle, therefore
they are different triangles; but they do not differ by a
differentia of figure, but are in one and the same division of it. For
a figure of the one kind is a circle and a figure of another kind of
triangle, and a triangle of one kind is equilateral and a triangle
of another kind scalene. They are the same figure, then, that,
triangle, but not the same triangle. Therefore the number of two
groups also-is the same number (for their number does not differ by
a differentia of number), but it is not the same decad; for the things
of which it is asserted differ; one group are dogs, and the other
horses.

    We have now discussed time-both time itself and the matters
appropriate to the consideration of it.