Let's assume: - we have standard civilization collapse (let's ignore the reasons, but assume that whatever the event was it devastated everything quickly and there are no serious additional lingering effects like significant fallout) - there is low amount of remaining people and they are somewhat scattered - there are few remaining artefacts (at least few working remaining artefacts) - there was a period (at least two generations) during which the majority had to become hunter gatherers by default Which technologies (ideas) would be easy to preserve? My first idea what would matter and is easy to preserve (even in long run as religious commandment / taboo) is hygiene. It would matter a lot, does not require high tech and everyone knows about it. Rhythm method? Alphabet??? (a bit trickier, but phonetic alphabet is useful and it is knowledge that practically everyone has) Evolution and Mendelian genetics? Contemporary astronomy (including big bang) preserved as equivalent of tribal creation myth? Democracy?/Codecs of law? (Of course in simplified version or at least mentioning it legends of perfect ;) past) Domestication of animals and plants? (or attempts to restore this procedure) Any other ideas?