sense that the dog moves while the bone remains at rest (until seized), and that the motion has a
purpose, namely to fulfil the dog's "nature." But it has turned out that this point of view cannot be
applied to dead matter, and that, for the purposes of scientific physics, no conception of an "end"
is useful, nor can any motion, in scientific strictness, be treated as other than relative.
Aristotle rejects the void, as maintained by Leucippus and Democritus. He then passes on to a
rather curious discussion of time. It might, he says, be maintained that time does not exist, since it
is composed of past and future, of which one no longer exists while the other does not yet exist.
This view, however, he rejects. Time, he says, is motion that admits of numeration. (It is not clear
why he thinks numeration essential.) We may fairly ask, he continues, whether time could exist
without the soul, since there cannot be anything to count unless there is some one to count, and
time involves numeration. It seems that he thinks of time as so many hours or days or years. Some
things, he adds, are eternal, in the sense of not being in time; presumably he is thinking of such
things as numbers.
There always has been motion, and there always will be; for there cannot be time without motion,
and all are agreed that time is uncreated, except Plato. On this point, Christian followers of
Aristotle were obliged to dissent from him, since the Bible tells us that the universe had a
beginning.
The Physics ends with the argument for an unmoved mover, which we considered in connection
with the Metaphysics. There is one unmoved mover, which directly causes a circular motion.
Circular motion is the primary kind, and the only kind which can be continuous and infinite. The
first mover has no parts or magnitude and is at the circumference of the world.
Having reached this conclusion, we pass on to the heavens.
The treatise On the Heavens sets forth a pleasant and simple theory. Things below the moon are
subject to generation and decay; from the moon upwards, everything is ungenerated and
indestructible. The earth, which is spherical, is at the centre of the universe. In the sublunary
sphere, everything is composed of the four elements, earth, water, air, and fire; but there is a fifth
element, of which the heavenly bodies are composed. The natural movement of the terrestrial
elements is rectilinear, but that of the fifth element is circular. The