Marco Polo describes a bridge, stone by stone. - "But which is the stone that supports the bridge?" asks Kublai Khan. - "The bridge is not supported by this or that stone," Marco answers, "but by the line of the arch that they form." Kublai Khan remains silent, reflecting. Then he adds: - " Why are you talking to me about stones? It is only the arch that matters to me." Polo responds: "Without stones, there is no arch."

                                                                                                                                (Invisible Cities, Italo Calvino)