In The Invention of Science: A New History of the Scientific Revolution, David Wootton describes how, prior to Columbus's discovery of the New World, Europeans believed that there was no new knowledge to be gained and that the ancient Greeks already knew everything. They knew the earth was round but many assumed their was only one landmass consisting of Europe, Asia, and Africa. (Paraphrasing and somewhat oversimplifying his argument.)

However, they should have known that Eratosthenes had calculated the circumference of the Earth. (We know now that it had an error of about 10-15%, but Columbus and his contemporaries wouldn't have known that.) Asia is much closer to Europe when traveling west to east. Why, then, would they have thought that it might be easier or faster to reach Asia by sailing west?