Physics   

Physics
By Aristotle
Written 350 B.C.E Part 1
Nature has been defined as a 'principle of motion and change', and it
is the subject of our inquiry. We must therefore see that we
understand the meaning of 'motion'; for if it were unknown, the
meaning of 'nature' too would be unknown.
When we have determined the nature of motion, our next task will be to
attack in the same way the terms which are involved in it. Now motion
is supposed to belong to the class of things which are continuous; and
the infinite presents itself first in the continuous-that is how it
comes about that 'infinite' is often used in definitions of the
continuous ('what is infinitely divisible is continuous'). Besides
these, place, void, and time are thought to be necessary conditions of
motion.
Clearly, then, for these reasons and also because the attributes
mentioned are common to, and coextensive with, all the objects of our
science, we must first take each of them in hand and discuss it. For
the investigation of special attributes comes after that of the common
attributes.
To begin then, as we said, with motion.
We may start by distinguishing (1) what exists in a state of
fulfilment only, (2) what exists as potential, (3) what exists as
potential and also in fulfilment-one being a 'this', another 'so
much', a third 'such', and similarly in each of the other modes of the
predication of being.
Further, the word 'relative' is used with reference to (1) excess and
defect, (2) agent and patient and generally what can move and what can
be moved. For 'what can cause movement' is relative to 'what can be
moved', and vice versa.
Again, there is no such thing as motion over and above the things. It
is always with respect to substance or to quantity or to quality or to
place that what changes changes. But it is impossible, as we assert,
to find anything common to these which is neither 'this' nor quantum
nor quale nor any of the other predicates. Hence neither will motion
and change have reference to something over and above the things
mentioned, for there is nothing over and above them.
Now each of these belongs to all its subjects in either of two ways:
namely (1) substance-the one is positive form, the other privation;
(2) in quality, white and black; (3) in quantity, complete and
incomplete; (4) in respect of locomotion, upwards and downwards or
light and heavy. Hence there are as many types of motion or change as
there are meanings of the word 'is'.
We have now before us the distinctions in the various classes of being
between what is full real and what is potential.
Def. The fulfilment of what exists potentially, in so far as it exists
potentially, is motion-namely, of what is alterable qua alterable,
alteration: of what can be increased and its opposite what can be
decreased (there is no common name), increase and decrease: of what
can come to be and can pass away, coming to he and passing away: of
what can be carried along, locomotion.
Examples will elucidate this definition of motion. When the buildable,
in so far as it is just that, is fully real, it is being built, and
this is building. Similarly, learning, doctoring, rolling, leaping,
ripening, ageing.
The same thing, if it is of a certain kind, can be both potential and
fully real, not indeed at the same time or not in the same respect,
but e.g. potentially hot and actually cold. Hence at once such things
will act and be acted on by one another in many ways: each of them
will be capable at the same time of causing alteration and of being
altered. Hence, too, what effects motion as a physical agent can be
moved: when a thing of this kind causes motion, it is itself also
moved. This, indeed, has led some people to suppose that every mover
is moved. But this question depends on another set of arguments, and