Our enquiry will resolve itself at the outset into a consideration
of the above-mentioned problem-what can be the reason why some
things in the world at one time are in motion and at another are at
rest again? Now one of three things must be true: either all things
are always at rest, or all things are always in motion, or some things
are in motion and others at rest: and in this last case again either
the things that are in motion are always in motion and the things that
are at rest are always at rest, or they are all constituted so as to
be capable alike of motion and of rest; or there is yet a third
possibility remaining-it may be that some things in the world are
always motionless, others always in motion, while others again admit
of both conditions. This last is the account of the matter that we
must give: for herein lies the solution of all the difficulties raised
and the conclusion of the investigation upon which we are engaged.

    To maintain that all things are at rest, and to disregard
sense-perception in an attempt to show the theory to be reasonable,
would be an instance of intellectual weakness: it would call in
question a whole system, not a particular detail: moreover, it would
be an attack not only on the physicist but on almost all sciences
and all received opinions, since motion plays a part in all of them.
Further, just as in arguments about mathematics objections that
involve first principles do not affect the mathematician-and the other
sciences are in similar case-so, too, objections involving the point
that we have just raised do not affect the physicist: for it is a
fundamental assumption with him that motion is ultimately referable to
nature herself.

    The assertion that all things are in motion we may fairly regard
as equally false, though it is less subversive of physical science:
for though in our course on physics it was laid down that rest no less
than motion is ultimately referable to nature herself, nevertheless
motion is the characteristic fact of nature: moreover, the view is
actually held by some that not merely some things but all things in
the world are in motion and always in motion, though we cannot
apprehend the fact by sense-perception. Although the supporters of
this theory do not state clearly what kind of motion they mean, or
whether they mean all kinds, it is no hard matter to reply to them:
thus we may point out that there cannot be a continuous process either
of increase or of decrease: that which comes between the two has to be
included. The theory resembles that about the stone being worn away by
the drop of water or split by plants growing out of it: if so much has
been extruded or removed by the drop, it does not follow that half the
amount has previously been extruded or removed in half the time: the
case of the hauled ship is exactly comparable: here we have so many
drops setting so much in motion, but a part of them will not set as
much in motion in any period of time. The amount removed is, it is
true, divisible into a number of parts, but no one of these was set in
motion separately: they were all set in motion together. It is
evident, then, that from the fact that the decrease is divisible
into an infinite number of parts it does not follow that some part
must always be passing away: it all passes away at a particular
moment. Similarly, too, in the case of any alteration whatever if that
which suffers alteration is infinitely divisible it does not follow
from this that the same is true of the alteration itself, which
often occurs all at once, as in freezing. Again, when any one has
fallen ill, there must follow a period of time in which his
restoration to health is in the future: the process of change cannot
take place in an instant: yet the change cannot be a change to
anything else but health. The assertion. therefore, that alteration is
continuous is an extravagant calling into question of the obvious: for
alteration is a change from one contrary to another. Moreover, we
notice that a stone becomes neither harder nor softer. Again, in the
matter of locomotion, it would be a strange thing if a stone could
be falling or resting on the ground without our being able to perceive
the fact. Further, it is a law of nature that earth and all other
bodies should remain in their proper places and be moved from them
only by violence: from the fact then that some of them are in their
proper places it follows that in respect of place also all things
cannot be in motion. These and other similar arguments, then, should
convince us that it is impossible either that all things are always in
motion or that all things are always at rest.

    Nor again can it be that some things are always at rest, others
always in motion, and nothing sometimes at rest and sometimes in
motion. This theory must be pronounced impossible on the same
grounds as those previously mentioned: viz. that we see the
above-mentioned changes occurring in the case of the same things. We
may further point out that the defender of this position is fighting
against the obvious, for on this theory there can be no such thing
as increase: nor can there be any such thing as compulsory motion,
if it is impossible that a thing can be at rest before being set in
motion unnaturally. This theory, then, does away with becoming and
perishing. Moreover, motion, it would seem, is generally thought to be
a sort of becoming and perishing, for that to which a thing changes
comes to be, or occupancy of it comes to be, and that from which a
thing changes ceases to be, or there ceases to be occupancy of it.
It is clear, therefore, that there are cases of occasional motion
and occasional rest.

    We have now to take the assertion that all things are sometimes at
rest and sometimes in motion and to confront it with the arguments
previously advanced. We must take our start as before from the
possibilities that we distinguished just above. Either all things
are at rest, or all things are in motion, or some things are at rest
and others in motion. And if some things are at rest and others in
motion, then it must be that either all things are sometimes at rest
and sometimes in motion, or some things are always at rest and the
remainder always in motion, or some of the things are always at rest
and others always in motion while others again are sometimes at rest
and sometimes in motion. Now we have said before that it is impossible
that all things should be at rest: nevertheless we may now repeat that
assertion. We may point out that, even if it is really the case, as
certain persons assert, that the existent is infinite and
motionless, it certainly does not appear to be so if we follow
sense-perception: many things that exist appear to be in motion. Now
if there is such a thing as false opinion or opinion at all, there
is also motion; and similarly if there is such a thing as imagination,
or if it is the case that anything seems to be different at
different times: for imagination and opinion are thought to be motions
of a kind. But to investigate this question at all-to seek a
reasoned justification of a belief with regard to which we are too
well off to require reasoned justification-implies bad judgement of
what is better and what is worse, what commends itself to belief and
what does not, what is ultimate and what is not. It is likewise
impossible that all things should be in motion or that some things
should be always in motion and the remainder always at rest. We have
sufficient ground for rejecting all these theories in the single
fact that we see some things that are sometimes in motion and
sometimes at rest. It is evident, therefore, that it is no less
impossible that some things should be always in motion and the
remainder always at rest than that all things should be at rest or
that all things should be in motion continuously. It remains, then, to
consider whether all things are so constituted as to be capable both
of being in motion and of being at rest, or whether, while some things
are so constituted, some are always at rest and some are always in
motion: for it is this last view that we have to show to be true.