buted to the constant conflict between heat and cold, and to
affinities of the particles: in this he was opposed to the doctrine of
Thales, Anaximenes, and Diogenes of Apollonia, who agreed in deriving all
things from a single, not _changeable_, principle.

Anaximander further held that the earth was of a cylindrical form,
suspended in the middle of the universe, and surrounded by water, air, and
fire, like the coats of an onion; but that the interior stratum of fire
was broken up and collected into masses, from which originated the sun,
moon, and stars; which he thought were carried round by the three spheres
in which they were respectively fixed. He believed that the moon had a
light of her own, not a borrowed light; that she was nineteen times as
large as the earth, and the sun twenty-eight. He thought that all animals,
including man, were originally produced in water, and proceeded gradually
to become land animals. According to Diogenes Laertius, he was the
inventor of the gnomon, and of geographical maps; at all events, he was
the first person who introduced the use of the gnomon into Greece. He died
about 547 B.C.

_Anaximenes_ was also a Milesian, and a contemporary of Thales and
Anaximander. We do not exactly know when he was born, or when he died; but
he must have lived to a very great age, for he was in high repute as early
as B.C. 544, and he was the tutor of Anaxagoras, B.C. 480. His theory was,
that air was the first cause of all things, and that the other elements of
the universe were resolvable into it. From this infinite air, he imagined
that all finite things were formed by compression and rarefaction,
produced by motion, which had existed from all eternity; so that the earth
was generated out of condensed air, and the sun and other heavenly bodies
from the earth. He thought also that heat and cold were produced by
different degrees of density of this primal element, air; that the clouds
were formed by the condensing of the air; and that it was the air which
supported the earth, and kept it in its place. Even the human soul he
believed to be, like the body, formed of air. He believed in the eternity
of matter, and denied the existence of anything immaterial.

_Anaxagoras_, who, as has been already stated, was a pupil of Anaximenes,
was born at Clazomenae, in Ionia, about B.C. 499. He removed to Athens at
the time of the Persian war, where he became intimate with Pericles, who
defended him, though unsuc