the conception of indivisible particles (+atomoi+), impenetrable
in their occupancy of space, and varying among themselves only in form,
order, and position. To provide for the motion that distributes them he
conceived them as separated from one another by empty space. From this
it follows that the void is as real as matter, or, as Democritus himself
is reputed to have said, "thing is not more real than no-thing."

But atomism has not been by any means universally regarded as the most
satisfactory conception of the relation between space and matter. Not
only does it require two kinds of being, with the different attributes
of extension and hardness, respectively,[229:3] but it would also seem
to be experimentally inadequate in the case of the more subtle physical
processes, such as light. The former of these is a speculative
consideration, and as such had no little weight with the French
philosopher Descartes, whose divisions and definitions so profoundly
affected the course of thought in these matters after the sixteenth
century. Holding also "that a vacuum or space in which there is
absolutely no body is repugnant to reason," and that an indivisible
space-filling particle is self-contradictory, he was led to _identify
space and matter_; that is, to make matter as indispensable to space as
space to matter. There is, then, but one kind of corporeal being, whose
attribute is extension, and whose modes are motion and rest. The most
famous application of the mechanical conceptions which he bases upon
this first principle, is his theory of the planets, which are conceived
to be embedded in a transparent medium, and to move with it, vortex
fashion, about the sun.[230:4]

But the conception of the space-filling continuity of material substance
owes its prominence at the present time to the experimental hypothesis
of _ether_. This substance, originally conceived to occupy the
intermolecular spaces and to serve as a medium for the propagation of
undulations, is now regarded by many physicists as replacing matter. "It
is the great hope of science at the present day," says a contemporary
exponent of naturalism, "that hard and heavy matter will be shown to be
ether in motion."[231:5] Such a theory would reduce bodies to the
relative displacements of parts of a continuous substance, which would
be first of all defined as spacial, and would possess such further
properties as special scientific hypotheses might require.

Two broadl