In the Culture's experience, when a certain class of society - a class which included nearly all humanoid societies - reached a certain critical point in their development, a complex decision had to be made as to its future direction. Such a society would be rapidly industrializing, often in a rather dirty and uncontrolled way, but nevertheless making great strides in the development of complex machines, smart systems and, crucially, spaceflight.
Such a society would move a significant part of their industrial base off-planet, to orbiting habitats and manufactories, moons and other worlds in their system, while striving to reduce the impact on their planet's ecosphere of their previous manufacturing activities, as well as improving the living conditions for the entire population. They would reach out further into space, using orbital telescopes and other instruments of great subtlety, to categorise and study the properties of other stars, other planets, to identify the possibility of life on other worlds, seeking Others and slowly coming to the realisation that they were Not Alone.
It was this path that the Culture's Contact section sought to promote, encourage, assist, mentor and direct a new civilization which might, in time, take its place in the greater galaxy amongst the wonderous splendours of the Involved.
But it was not the only possible path; it was just one of several paths a newly emerging intelligent species could take.
An industrial civilization on a planet could regress. This could be as a result of a war, a global conflict which destroyed much of the infrastructure, poisoned much of the earth and the water, and left the remaining survivors suddenly reduced to a primitive state. Much the same result might be achieved by a plague or pandemic disease, or a military deployment of a virulent biological agent. Famine and exposure would result in population collapse to a thousandth or less of the previous level, tending to gather in a few widely scattered locations far from the previous centres. Some kind of new social organisation would arise and the survivors might typically endure for a few generations before dying out completely or, more rarely, form a stable low-tech civilization.
Sometimes, however, the regression to a more primitive society was a deliberate, planned affair. Some species never invented technology capable of proving the existence of planets around other stars - or chose not to invest in developing it - with the concomitant possibility of demonstrating there was life of any kind elsewhere. By chance - or some obscure design of the more recalcitrant members of the galactic community - they were never contacted by a space-faring civilization - as the Contact section's mission was to do in as many cases as possible - or were otherwise mentored towards high-level engagement in the affairs of the galaxy.
Faced with the realization - or at least assertion - that there was Nobody Out There, it was perhaps inevitable that some combination of religion and self-preservation would cause the society to turn inwards, to look at ways at sustaining their species on their planet indefinitely. Research would rapidly reveal that their planet could not sustain the use of resources at the rate they were presently being used, and therefore drastic actions would be required over a large number of generations. A dramatic reduction in population size - basically, enforced birth control so that only a tiny fraction of people could become parents - together with an aggressive program of re-wilding much of the planet's surface and careful removal of all of the industrial infrastructure.
These changes were typically combined with a return to a rural agrarian lifestyle - farming for cereals and vegetables, orchards and gardens for fruits and herbs, husbanding a few animals for leather and meat and muscles in the fields - as well as very simple industrial processes: smelting iron for tools and reinforcing parts, spinning and weaving vegetable or animal materials into cloth. It was a deliberately introduced statis, an artificial stability, intended to use some minuscule part of the renewable resources of the planet while keeping the vast majority of the surface in a stable state.
The Culture knew this as the Gaia Principle.