An example or two will make this unique human process clear.

Galileo was a thoughtful youth and doubtless carried on a rich and
varied reverie. He had artistic ability and might have turned out to
be a musician or painter. When he had dwelt among the monks at
Valambrosa he had been tempted to lead the life of a religious. As a
boy he busied himself with toy machines and he inherited a fondness
for mathematics. All these facts are of record. We may safely assume
also that, along with many other subjects of contemplation, the Pisan
maidens found a vivid place in his thoughts.

One day when seventeen years old he wandered into the cathedral of his
native town. In the midst of his reverie he looked up at the lamps
hanging by long chains from the high ceiling of the church. Then
something very difficult to explain occurred. He found himself no
longer thinking of the building, worshipers, or the services; of his
artistic or religious interests; of his reluctance to become a
physician as his father wished. He forgot the question of a career and
even the _graziosissime donne_. As he watched the swinging lamps he
was suddenly wondering if mayhap their oscillations, whether long or
short, did not occupy the same time. Then he tested this hypothesis by
counting his pulse, for that was the only timepiece he had with him.

This observation, however remarkable in itself, was not enough to
produce a really creative thought. Others may have noticed the same
thing and yet nothing came of it. Most of our observations have no
assignable results. Galileo may have seen that the warts on a
peasant's face formed a perfect isosceles triangle, or he may have
noticed with boyish glee that just as the officiating priest was
uttering the solemn words, _ecce agnus Dei_, a fly lit on the end of