But we must not forget that in the centres of civilization all along there were many men of powerful intellect.
Indeed, it would violate the principle of historical continuity to suppose that there was any sudden change in the level of mentality of the Roman world at the close of the classical period.

We must assume, then, that the direction in which the great minds turned was for some reason changed.

Newton is said to have alleged that he made his discoveries by "intending" his mind in a certain direction continuously.
It is probable that the same explanation may be given of almost every great scientific discovery.

Anaxagoras could not have thought out the theory of the moon's phases; Aristarchus could not have found out the true mechanism of the solar system; Eratosthenes could not have developed his plan for measuring the earth, had not each of these investigators "intended" his mind persistently towards the problems in question.
Nor can we doubt that men lived in every generation of the dark age who were capable of creative thought in the field of science, bad they chosen similarly to "intend" their minds in the right direction.

The difficulty was that they did not so choose.