And it must not be forgotten that his enterprise, as compared with that of the Portuguese along the coast of Africa, was as an invention compared to an improvement.
Each new discovery then was but a step beyond that which had preceded it; Columbus was the first to steer boldly from shore into the waste of waters, an originator, not a mere improver.
COLUMBUS'S THEORY.
Fernando Columbus divides into three classes the grounds on which his father's theory was based; namely, reasons from nature, the authority of writers, and the testimony of sailors.

He believed the world to be a sphere; he under-estimated its size; he over-estimated the size of the Asiatic continent.

The farther that continent extended to the eastward the nearer it came round towards Spain.

And this, in a greater or less degree, had been the opinion of the ancient geographers.

Both Aristotle and Seneca thought that a ship might sail "in a few days" from Cadiz to India.
Strabo, too, believed that it might be possible to navigate on the same parallel of latitude, due west from the coast of Africa or Spain to that of India.