Plato, Phaedo

[109a] nor any other similar force to keep it from falling, but its own equipoise and the homogeneous nature of the heavens on all sides suffice to hold it in place; for a body which is in equipoise and is placed in the center of something which is homogeneous cannot change its inclination in any direction, but will remain always in the same position. This, then, is the first thing of which I am convinced.”

“And rightly,” said Simmias.

“Secondly,” said he, “I believe that the earth is very large and that we who dwell between the pillars of Hercules [109b] and the river Phasis live in a small part of it about the sea, like ants or frogs about a pond, and that many other people live in many other such regions. For I believe there are in all directions on the earth many hollows of very various forms and sizes, into which the water and mist and air have run together; but the earth itself is pure and is situated in the pure heaven in which the stars are, the heaven which [109c] those who discourse about such matters call the ether; the water, mist and air are the sediment of this and flow together into the hollows of the earth. Now we do not perceive that we live in the hollows, but think we live on the upper surface of the earth, just as if someone who lives in the depth of the ocean should think he lived on the surface of the sea, and, seeing the sun and the stars through the water, should think the sea was the sky, and should, by reason of sluggishness or [109d] feebleness, never have reached the surface of the sea, and should never have seen, by rising and lifting his head out of the sea into our upper world, and should never have heard from anyone who had seen, how much purer and fairer it is than the world he lived in. I believe this is just the case with us; for we dwell in a hollow of the earth and think we dwell on its upper surface; and the air we call the heaven, and think that is the heaven in which the stars move. But the fact is the same, [109e] that by reason of feebleness and sluggishness, we are unable to attain to the upper surface of the air; for if anyone should come to the top of the air or should get wings and fly up, he could lift his head above it and see, as fishes lift their heads out of the water and see the things in our world, so he would see things in that upper world; and, if his nature were strong enough to bear the sight, he would recognize that that is the real heaven