I have been taught in heaven that the earliest people on our planet, who were heavenly people, thought on the basis of actual correspondences, and that the natural phenomena of the world that greeted their eyes served them as means for thinking in this way. Because they were of this character, they were in the company of angels and talked with them; and in this way heaven was united to the world through them. Therefore, that era was called the Golden Age. Classical authors described it as a time when the inhabitants of heaven dwelt with mortals and kept them company as friend with friend.
After their era, though, a people came who did not think from actual correspondences but from a knowledge about correspondences. There was still a union of heaven with humanity, but not such an intimate one. Their era was called the Silver Age.
The people who came next were indeed familiar with correspondences but did not do their thinking on the basis of their knowledge of correspondences. This was because they were engrossed in what is good on the natural level and not, like their ancestors, on the spiritual level. Their era was called the Bronze Age.
I have been taught, lastly, that after that era humanity became more and more externally minded and at last physically minded. Then the knowledge of correspondences was completely lost, and with it any awareness of heaven and of its riches. The names of these ages—Golden, Silver, and Bronze—also come from correspondence, because gold, by reason of correspondence, means the heavenly goodness in which the earliest people lived. Silver, in contrast, means the spiritual goodness in which their successors, the early people, lived; while bronze means the natural goodness characteristic of their immediate followers. Iron, though, which gave its name to the last era, means a harsh truth, devoid of good.
from Heaven and Hell, Section 115