one would define void as place bereft of body." This view is set forth with the utmost explicitness
by Newton, who asserts the existence of absolute space, and accordingly distinguishes absolute
from relative motion. In the Copernican controversy, both sides (however little they may have
realized it) were committed to this view, since they thought there was a difference between saying
"the heavens revolve from east to west" and saying "the earth rotates from west to east." If all
motion is relative, these two statements are merely different ways of saying the same thing, like
"John is the father of James" and "James is the son of John." But if all motion is relative, and
space is not substantial, we are left with the Parmenidean arguments against the void on our
hands.
Descartes, whose arguments are of just the same sort as those of early Greek philosophers, said
that extension is the essence of matter, and therefore there is matter everywhere. For him,
extension is an adjective, not a substantive; its substantive is matter, and without its substantive it
cannot exist. Empty space, to him, is as absurd as happiness without a sentient being who is
happy. Leibniz, on somewhat different grounds, also believed in the plenum, but he maintained
that space is merely a system of relations. On this subject there was a famous controversy between
him and Newton, the latter represented by Clarke. The controversy remained undecided until the
time of Einstein, whose theory conclusively gave the victory to Leibniz.
The modern physicist, while he still believes that matter is in some sense atomic, does not believe
in empty space. Where there is not matter, there is still something, notably light-waves. Matter no
longer has the lofty status that it acquired in philosophy through the arguments of Parmenides. It is
not unchanging substance, but merely a way of grouping events. Some events belong to groups
that can be regarded as material things; others, such as light-waves, do not. It is the events that are
the stuff of the world, and each of them is of brief duration. In this respect, modern physics is on
the side of Heraclitus as against Parmenides. But it was on the side of Parmenides until Einstein
and quantum theory.
As regards space, the modern view is that it is neither a substance, as Newton maintained, and as
Leucippus and Democritus ought to have said, nor an adjective of extended bodies, as Descartes
thought,