But Bede, in the eighth century, established it finally as a part of human knowledge that the earth and all the heavenly bodies were spheres, and after that the fact was not again seriously disputed.
What lay beyond the frontier of the known was a speculation inseparable from the spirit of exploration.

Children, and people who do not travel, are generally content, when their thoughts stray beyond the paths trodden by their feet, to believe that the greater world is but a continuation on every side of their own environment; indeed, without the help of sight or suggestion, it is almost impossible to believe anything else.

If you stand on an eminence in a great plain and think of the unseen country that lies beyond the horizon, trying to visualise it and imagine that you see it, the eye of imagination can only see the continuance or projection of what is seen by the bodily sight.

If you think, you can occupy the invisible space with a landscape made up from your own memory and knowledge: you may think of mountain chains and rivers, although there are none visible to your sight, or you may imagine vast seas and islands, oceans and continents.

This, however, is thought, not pure imagination; and even so, with every advantage of thought and knowledge, you will not be able to imagine beyond your horizon a space of sea so wide that the farther shore is invisible, and yet imagine the farther shore also.