We know that
Tycho-Brahe, lacking inventive genius but rich in capacity for exact
observation, met Kepler, an adventurous spirit: together, the two made a
complete scientist. We have seen how Kepler, guided by a preconceived
notion of the "harmony of the spheres," after many trials and
corrections, ended by discovering his laws. Copernicus recognized
expressly that his theory was suggested to him by an hypothesis of
Pythagoras--that of a revolution of the earth about a central fire,
assumed to be in a fixed position. Newton imagined his hypothesis of
gravitation from the year 1666 on, then abandoned it, the result of his
calculations disagreeing with observation; finally he took it up again
after a lapse of a few years, having obtained from Paris the new measure
of the terrestrial meridian that permitted him to prove his guess. In
relating his discoveries, Lavoisier is lavish in expressions that leave
no doubt as to their originally conjectural character. "He _suspects_
that the air of the atmosphere is not a simple thing, but is composed of
two very different substances." "He _presumes_ that the permanent
alkalies (potash, soda) and the earths (lime, magnesia) should not be
considered simple substances." And he adds: "What I present here is at
the most no more than a mere _conjecture_." We have mentioned above the
case of Darwin. Besides, the history of scientific discoveries is full
of facts of this sort.

The passage from the imaginative to the rational phase may be slow or
sudden. "For eight months," says Kepler, "I have seen a first glimmer;
for three months, daylight; for the last week I see the sunlight of the
most wonderful contemplation." On the other hand, Hauey drops a bit of
crystallized calcium spar, and, looking at one of the broken prisms,
cries out, "All is found!" and immediately verifies his quick intuition
in regard to the true nature of crystallization. We have already
indicated[114] the psychological reasons for these differences.

Underneath all the reasoning, inductions, deductions, calculations,
demonstrations, methods, and logical apparatus of every sort, there is
something animating them that is not understood, that is the work of
that complex operation--the constructive imagination.

To conclude: The hypothesis is a creation of the mind, invested with a
provisional reality that may, after verification, become permanent.
False hypotheses are characterized as imaginary, by which designation i