Sigmund Freud, in his second to last book, chose Rome as the metaphor to discuss a new idea: The proposition that when the mind forgets nothing is really lost. According to the great Austrian theorist, what we experience as forgotten remains hidden beneath new layers of experience:
We will chose as an example the history of the Eternal City. Historians tell us that the oldest Rome was the Roma Quadrata, a fenced settlement on the Palatine. Then followed the phase of Septimontium, a federation of the settlements on the [seven] different hills; after that came the city bounded by the Servian Wall; and later still, after all the transformations during the periods of the republic and the early Caesars, the city which the Emperor Aurelian surrounded with his walls. We will not follow the changes which the city went through any further, but we will ask ourselves how much a visitor, whom we will suppose to be equipped with the most complete historical and topographical knowledge, may still find left of these early stages in the Rome of today. Except for a few gaps, he will see the wall of Aurelian almost unchanged. In some places he will be able to find sections of the Servian wall where they have been excavated and brought to light. If he knows enough—more than present-day archaeology does—he may perhaps be able to trace out in the plan of the city the whole course of that wall and outline of the Roma Quadrata. Of the buildings which once occupied this ancient area he will find nothing, or only scanty remains, for they exist no longer. The best information about Rome in the republican era would only enable him at the most to point out the sites where the temples and public buildings of that period stood. Their place is now taken by ruins, but not by ruins of themselves but of later restorations made after fires or destruction. It is hardly necessary to remark that all these remains of ancient Rome are found dovetailed into the jumble of a great metropolis which has grown up in the last few centuries since the Renaissance. There is certainly not a little that is ancient still buried in the soil of the city or beneath its modern buildings. This is the manner in which the past is preserved in historical sites like Rome.
Civilization and its Discontents, pp. 6-7
Writing in 1930, the author of Das Unbehagen in der Kultur experienced Rome almost without cars. Yet, vehicular pollution, the vibration imparted on the ancient monuments by the friction of rubber tires on pavement, and the soot that washes over new and old stones, is not necessarily a fate worse than what greeted Romans in antiquity. Trading carbon-monoxides for the dung and straw of streets full of horse and mule pulled wagons, defecating and urinating in the public rights of way, may not be a bad bargain after all. Historians of the period report a Rome complete with rush hours and standing still congestion twice a day, morning and afternoon. Julius Ceasar banned wheeled traffic from the center of Rome during the day, inadvertently causing an insufferable racket at night. Emperor Hadrian finally put a limit on the number of teams of horses and loads of carts permitted to enter the city altogether (AD 117-138, builder of the Pantheon).
Today we see little of the achievement that was the ancient city. More than 1800 years would pass before London would be the first western metropolis to match its 1 million inhabitants. It would not be until the 20th century that Chicago—the city by the lake—would deliver an equivalent daily flow of fresh water to match the capacity of the 11 aqueducts feeding the Eternal City. A municipality has yet to match the program of free bread to its citizens that made Egypt the bread basket of the Capital. Considering the annual tonnage of grain, the importation of 13 Egyptian obelisks amidst the grain shipments seems matter of course. Even if this legacy were visible today, it would be no less comprehensible to our modern eyes. We still lack a theory capable of explaining the essential bits—including the gore—that made up ancient Rome. A place enduring most of our urban problems without any of our modern means.