Physics   

It is not difficult to see how each of its types will be
defined-alteration is the fulfillment of the alterable qua alterable
(or, more scientifically, the fulfilment of what can act and what can
be acted on, as such)-generally and again in each particular case,
building, healing, &c. A similar definition will apply to each of the
other kinds of motion.
Part 4
The science of nature is concerned with spatial magnitudes and motion
and time, and each of these at least is necessarily infinite or
finite, even if some things dealt with by the science are not, e.g. a
quality or a point-it is not necessary perhaps that such things should
be put under either head. Hence it is incumbent on the person who
specializes in physics to discuss the infinite and to inquire whether
there is such a thing or not, and, if there is, what it is.
The appropriateness to the science of this problem is clearly
indicated. All who have touched on this kind of science in a way worth
considering have formulated views about the infinite, and indeed, to a
man, make it a principle of things.
(1) Some, as the Pythagoreans and Plato, make the infinite a principle
in the sense of a self-subsistent substance, and not as a mere
attribute of some other thing. Only the Pythagoreans place the
infinite among the objects of sense (they do not regard number as
separable from these), and assert that what is outside the heaven is
infinite. Plato, on the other hand, holds that there is no body
outside (the Forms are not outside because they are nowhere),yet that
the infinite is present not only in the objects of sense but in the
Forms also.
Further, the Pythagoreans identify the infinite with the even. For
this, they say, when it is cut off and shut in by the odd, provides
things with the element of infinity. An indication of this is what
happens with numbers. If the gnomons are placed round the one, and
without the one, in the one construction the figure that results is
always different, in the other it is always the same. But Plato has
two infinites, the Great and the Small.
The physicists, on the other hand, all of them, always regard the
infinite as an attribute of a substance which is different from it and
belongs to the class of the so-called elements-water or air or what is
intermediate between them. Those who make them limited in number never
make them infinite in amount. But those who make the elements infinite
in number, as Anaxagoras and Democritus do, say that the infinite is
continuous by contact-compounded of the homogeneous parts according to
the one, of the seed-mass of the atomic shapes according to the other.
Further, Anaxagoras held that any part is a mixture in the same way as
the All, on the ground of the observed fact that anything comes out of
anything. For it is probably for this reason that he maintains that
once upon a time all things were together. (This flesh and this bone
were together, and so of any thing: therefore all things: and at the
same time too.) For there is a beginning of separation, not only for
each thing, but for all. Each thing that comes to be comes from a
similar body, and there is a coming to be of all things, though not,
it is true, at the same time. Hence there must also be an origin of
coming to be. One such source there is which he calls Mind, and Mind
begins its work of thinking from some starting-point. So necessarily
all things must have been together at a certain time, and must have
begun to be moved at a certain time.
Democritus, for his part, asserts the contrary, namely that no element
arises from another element. Nevertheless for him the common body is a
source of all things, differing from part to part in size and in
shape.
It is clear then from these considerations that the inquiry concerns
the physicist. Nor is it without reason that they all make it a
principle or source. We cannot say that the infinite has no effect,
and the only effectiveness which we can ascribe to it is that of a
principle. Everything is either a source or derived from a source. But