can never be changed, but can only be mixed
together to form substances of the material world, we are not reading
back post-Daltonian knowledge into the system of Anaxagoras. Here are
his words: "The Greeks do not rightly use the terms 'coming into being'
and 'perishing.' For nothing comes into being, nor, yet, does anything
perish; but there is mixture and separation of things that are. So they
would do right in calling 'coming into being' 'mixture' and 'perishing'
'separation.' For how could hair come from what is not hair? Or flesh
from what is not flesh?"

Elsewhere he tells us that (at one stage of the world's development)
"the dense, the moist, the cold, the dark, collected there where now
is earth; the rare, the warm, the dry, the bright, departed towards the
further part of the aether. The earth is condensed out of these things
that are separated, for water is separated from the clouds, and earth
from the water; and from the earth stones are condensed by the cold, and
these are separated farther from the water." Here again the influence of
heat and cold in determining physical qualities is kept pre-eminently in
mind. The dense, the moist, the cold, the dark are contrasted with the
rare, the warm, the dry, and bright; and the formation of stones is
spoken of as a specific condensation due to the influence of cold. Here,
then, we have nearly all the elements of the Daltonian theory of atoms
on the one hand, and the nebular hypothesis of Laplace on the other. But
this is not quite all. In addition to such diverse elementary particles
as those of gold, water, and the rest, Anaxagoras conceived a species of
particles differing from all the others, not merely as they differ
from one another, but constituting a class by themselves; particles
infinitely smaller than the others; particles that are described as
infinite, self-powerful, mixed with nothing, but existing alone. That is
to say (interpreting the theory in the only way that seems plausible),
these most minute particles do not mix with the other primordial
particles to form material substances in the same way in which these
mixed with one another. But, on the other hand, these "infinite,
self-powerful, and unmixed" particles commingle everywhere and in every
substance whatever with the mixed particles that go to make up the
substances.

There is a distinction here, it will be observed, which at once
suggests the modern distinction between physical processes an