.]

[Footnote 789: Diogenes Laertius, "Lives of the Philosophers," bk. x.
ch. xxiv.]

The two great principles of nature are a _vacuum_, and a _plenum._ The
plenum is _body_, or tangible nature; the vacuum is _space_, or
intangible nature. "We know by the evidences of the senses (which are
our only rule of reasoning) that _bodies_ have a real existence, and we
infer from the evidence of the senses that the vacuum has a real
existence; for if space have no real existence, there would be nothing
in which bodies can move, as we see they really do move. Let us add to
this reflection that one can not conceive, either in virtue of
perception, or of any analogy founded on perception, any general quality
peculiar to all beings, which is not either an attribute, or an
accident, of the body or of the vacuum."[790]

Of bodies some are "combinations"--concrete bodies--and some are
primordial "elements," out of which combinations are formed. These
primordial elements, out of which the universe is generated, are
"_atoms_" (atomoi). These atoms are "the first principles" and "seeds"
of all things.[791] They are "_infinite_ in number," and, as their name
implies, they are "_infrangible" "unchangeable_" and
"_indestructible."_[792] Matter is, therefore, not infinitely divisible;
there must be a point at which division ends.[793]

The only qualities of atoms are _form_, _magnitude_, and _density._ All
the other sensible qualities of matter--the secondary qualities--as
color, odor, sweetness, bitterness, etc.--are necessarily inherent in
form. All secondary qualities are changeable, but the primary atoms are
unchangeable; "for in the dissolution of combined bodies there must be
something _solid_ and _indestructible,_ of such a kind that it will not
change, either into what does not exist, or out of what does not exist,
but the change results from a simple displacement of parts, which is the
most usual case, or from an addition or subtraction of particles."[794]

[Footnote 790: Diogenes Laertius, "Lives of the Philosophers," bk. x.
ch. xxiv.]

[Footnote 791: Id., ib., bk. x. ch. xxv.]

[Footnote 792: Id., ib., bk. x. ch. xxiv.]

[Footnote 793: Id., ib.; Lucretius, "On the Nature of Things," bk. i. l.
616-620.]

[Footnote 794: Diogenes Laertius, "Lives of the Philosophers," bk. x.
ch. xxiv.]

The atoms are not all of one _form_, but of different forms suited to
the production of different substances by combination; some are squ