fifteenth of May with fresh energy,--it rose at last above the darkness
of my understanding, so fully confirmed by my labor of seventeen years
upon Brahe's observations, and by my own meditations perfectly agreeing
with them, that I thought at first I was dreaming, and making some
_petitio principii_; but there is no more doubt about it: it is a very
certain and very exact proposition."[164]

All the logic of discoveries is laid down in these lines; and these
lines are a testimony rendered by one of the most competent of
witnesses. You see in them the conditions of a good hypothesis: Kepler
has long studied the phenomena of which he wishes to find the law; he
has studied them by himself, and by means of the discoveries of his
predecessor Brahe. The law has presented itself to his mind at a given
moment, on the eighth of March, 1618. But he does not yet know whether
it is a true light, or a deceptive gleam. He seeks the confirmation of
his hypothesis; he does not find it, because he makes a mistake, and he
rejects his idea as useless. The idea returns; a new course of labor
confirms it; and so the hypothesis becomes a law, a certain proposition.

Such is the regular march of thought. An hypothesis has no right to be
brought forward until it has passed into the condition of a law, by
being duly confirmed. There are minds, however, endowed with a sort of
divination, which feel as by instinct the truth of a discovery, even
before it has been confirmed. It is told of Copernicus, that having
discovered, or re-discovered, the true system of planetary motion, he
encountered an opponent who said to him: "If your system were true,
Venus would have phases like the moon; now she has none, and therefore
your system is false. What have you to reply?"--"I have no reply to
make," said Copernicus, (the objection was a serious one in fact); "but
God will grant that the answer shall be found."[165] Galileo appeared,