n promulgated without hinderance, because it was deemed allowable to
dispute concerning natural things, and to elucidate the works of God,
and now that new testimony is discovered in proof of the truth of those
doctrines--testimony which was not known to the spiritual judges--ye
would prohibit the promulgation of the true system of the structure of
the universe."

None of Kepler's contemporaries believed the law of the areas, nor was
it accepted until the publication of the "Principia" of Newton. In fact,
no one in those times understood the philosophical meaning of Kepler's
laws. He himself did not foresee what they must inevitably lead to. His
mistakes showed how far he was from perceiving their result. Thus he
thought that each planet is the seat of an intelligent principle, and
that there is a relation between the magnitudes of the orbits of the
five principal planets and the five regular solids of geometry. At first
he inclined to believe that the orbit of Mars is oval, nor was it until
after a wearisome study that he detected the grand truth, its elliptical
form. An idea of the incorruptibility of the celestial objects had
led to the adoption of the Aristotelian doctrine of the perfection of
circular motions, and to the belief that there were none but circular
motions in the heavens. He bitterly complains of this as having been a
fatal "thief of his time." His philosophical daring is illustrated in
his breaking through this time-honored tradition.

In some most important particulars Kepler anticipated Newton. He was the
first to give clear ideas respecting gravity. He says every particle of
matter will rest until it is disturbed by some other particle--that the
earth attracts a stone more than the stone attracts the earth, and that
bodies move to each other in proportion to their masses; that the earth
would ascend to the moon one-fifty-fourth of the distance, and the moon
would move toward the earth the other fifty-three. He affirms that the
moon's attraction causes the tides, and that the planets must impress
irregularities on the moon's motions.

The progress of astronomy is obviously divisible into three periods:

1. The period of observation of the apparent motions of the heavenly
bodies.

2. The period of discovery of their real motions, and particularly of
the laws of the planetary revolutions; this was signally illustrated by
Copernicus and Kepler.

3. The period of the ascertainment of the causes of