Such a state had never been seen, and therefore could not be conceived by them.

Their experience (Aristot.

Metaph.; Plato, Laws) led them to conclude that there had been cycles of civilization in which the arts had been discovered and lost many times over, and cities had been overthrown and rebuilt again and again, and deluges and volcanoes and other natural convulsions had altered the face of the earth.

Tradition told them of many destructions of mankind and of the preservation of a remnant.
The world began again after a deluge and was reconstructed out of the fragments of itself.

Also they were acquainted with empires of unknown antiquity, like the Egyptian or Assyrian; but they had never seen them grow, and could not imagine, any more than we can, the state of man which preceded them.