But this can be applied to the world at large as well as the mental cosmos. Perhaps not perfectly (for what mental construct can translate without some changes into the real world?) but enough to suit our purposes. We can think through memory here to come to an understanding of ruinous signs and what they portend or foretell.
We must first take stock of the cycles of the world: great wealth and sophistication followed by social decay and ruin. These cycles are not unavoidable, but they have happened in the past. When society suffers decay for whatever reason, technological advances are often lost, particularly in those societies that have heavily specialized means of production (like Rome, for example). During this process, active living places are transformed into ruins. The ruins, however, serve to trigger a memorial sequence that reminds us of what once stood there... as long as we know what the ruins were meant to represent.
As time goes on and the original generations who lived within the memorial-ruins give way to others, the sequence continues pointing to the past... but the construction of the past deviates further and further from the experiences that formed it. Eventually, it may give way altogether: the past as alien construct which the viewer cannot understand. Thus, extremely ancient ruins give way to entirely fictive pasts constructed to explain them while ruins that were made nearer to the viewer's own time period tend to have more or less "grounded" connections.
But ruins are something special; they are not just the remains left behind by other civilizations, they are living semions, signs whose meaning and signification evolve over time. They can point the way to strange and mysterious pasts (as archaeological study would have it) or they can be the foundation of completely new constructions. The most important thing is that the meaning or purpose of the original ruin may not always be clear, sensible, or intelligible. While it is interesting indeed to think of why a thing was made when we design it (and it brings a good deal of flavor to it) we can design ruins that have no purpose, or that had a purpose but that those now in them could not figure out.