The bird is not a higher form of imagination than we are, but its ability to fly symbolizes one, and men usually assign wings to what they visualize as superior forms of human existence. In this symbolism the corresponding image of nature would be neither the seed-bed of the plant nor the suckling mother of the mammal, but the egg, which has been used as a symbol of the physical universe from the most ancient times…. In Blake the firmament is the Mundane Shell, the indefinite circumference of the physical world through which the mind crashes on its winged ascent to reality. To the inexperienced eye the egg appears to be a geometrical stone, but the imagination within the egg soon demonstrates that it is something much more fragile. The same is true of the Newtonian universe, the rock rolled against the tomb of divine humanity.

Northrop Frye, Fearful Symmetry, in Norman O. Brown, Love’s Body