I luv this allegory as an example of relating this world and the spritual world. copied and pasted from Republic)
Perhaps the most famous allegory in Republic is the Allegory of the Cave at the beginning of Bk. VI:
"Imagine human beings living an underground, cavelike dwelling, with an entrance a long way up, which is both open to the light and as wide as the cave itself. They've been there since childhood, fixed in the same place, with their necks and legs fettered, able to see only in front of them, because their bonds prevent them from turning their heads around. Light is provided by a fir burning far above and behind them. Also behind them, but on higher ground, there is a path stretching between them and the fire. Imagine that along this path a low wall has been built, like the screen in front of puppeteers above which they show their puppets.... also imagine that there are people along the wall, carrying all kinds of artifacts that project above it - statues of people and animals, made out of stone, wood, and every material. And, as you'd expect, some of the carriers are talking, some are silent." (p. 436-7)
The point of the allegory is to compare the experience of those who are not philosophers and who have never had exposure to the Forms (i.e. who have never left the Cave), with the experience of the philosophers who have had exposure to the Forms (i.e. who have left the Cave).
Those in the Cave only see shadows (or copies) of the artifacts carried behind them, thrown by the fire. The artifacts are themselves copies of things that exist outside of the Cave (animals, etc.) Since those in the Cave are unable to turn around, they take mere shadows of copies of originals to be what truly exists. Hence they are twice removed from the truth. This is the condition of those who have not been exposed to the Forms as the philosophers have.