I have tended to assume that common sense is something which pretty much everybody has - excepting the intellectual elite and psychotics - but which in our modern mass media has overlaid and overwhelmed.
But this is to take common sense for granted. Common sense is in fact a complex functional adaptation to (something like) 'survive and reproduce and raise offspring on earth', and complex adaptations can be lost. Indeed there is an inbuilt tendency to lose them.
Insofar as common sense is a gift of God; it can be lost by rejection of God. Insofar as common sense is a Good, it can be lost by subversion, inversion and the destruction of Good.
Insofar as common sense is a suite of evolved adaptations it can be destroyed by loss of adaptations - from mutational damage to the genes which make the structures which implement the common sense.
(All this on top of the fact that what was common sense in the ancestral evolutionary environment may be mis-matched to the modern environment and may function as common nonsense.)
So, until recently, I have assumed that when the mass media and the crazed, self-hating and suicidal structure of Leftism destroys itself or is destroyed, and brings down the material structure of modernity - we will 'at least' be left with common sense to help survival and rebuilding. Common sense and common experience - in a phrase I used.
But maybe I was being over-optimistic (!).
Common experience depends on intelligence, and average intelligence is falling so rapidly that it seems likely that people after the collapse will not able to learn from experience in the way that was normal through most of the past millennium or so: they will not be as quick nor as able to learn.
And if widespread and cumulatively-severe genetic damage from mutation accumulation is a major element in the decline of intelligence, as now seems to me (unfortunately) highly likely, then common sense will be much weakened and destroyed to a lesser or greater extent.
This seems inevitable as a consequence of accumulation of deleterious mutations which get worse with each generation, and which act like a shotgun blast against the genome - each 'pellet' smashing a gene, and the mass of pellets breaking functions here and there. Each new generation inherits the genetic damage and adds more damage.
The key point here (as Michael A Woodley realized in our conversation yesterday^) is that in mutation accumulation it is not only adaptations which are transmitted from the parent and inherited by offspring, but non-adaptations, broken adaptations, wrecked and useless remnants of adaptations.
Parents do not just transmit 'fitness' to their offspring, but also the accumulation of unfitness. Common sense is - or was - presumably, to some significant extent, a suite of interconnected and mutually-supportive adaptations supported by genes; and therefore damage to common sense may have a genetic cause, and will be heritable.
The 'nightmare scenario' would therefore be a human population which not only lacks the intelligence to sustain and operate complex modern civilization, but lacks the common sense to function even in simpler societies.
The main message is that although common sense used to be near universal and something which could be taken for granted - and which would re-emerge in situations of crisis - this is much too complacent a view. We cannot just assume that the people who survive crisis or collapse and re-emerge from the rubble will be armed with the levels of intelligence and common sense which we take for granted from history. Common sense is vulnerable, at the biological level.
(^Note: Michael Woodley's insight yesterday concerned the difference between on the one hand heredity of functional and fitness-enhancing adaptations, which has always been the focus of evolutionary theory, and the way that modernity has selected-for non-adaptation, for broken adaptations. And furthermore, damage, hence dysfunction, is not merely inherited but is added-to with each new generation - with each generational wave of new and un-purged mutational damage, with each failure to eliminate the cluster spontaneously occurring deleterious mutations. This is not a matter of offspring inheriting fitness from their parents, but inheriting 'anti-fitness'; and it is not only inherited - because offspring will differ from their parents in having novel and added and extra forms of heritable anti-fitness. In is not an adapted genome which is being selected-for, but a broken genome. In sum, modernity has not selected for a population who are 'adapted' to modern conditions; but for a population whose adaptations are being destroyed, progressively and generation upon generation. Modernity has not selected for fitness, not even for a new kind of fitness - but modernity has selected for a lack of fitness - for the destruction of complex functional adaptations. And this loss of fitness, of adaptations, is objective and in principle measurable and quantifiable. Such a situation may be unique to human society in the modern world - where humans created an artificial environment which destroyed a critical mechanism for purging deleterious mutations from each generation, and therefore both allowed the survival off, and also positively selected in favour of, high-mutation-load humans who could not sustain that artificial environment which allowed them to survive and reproduce. Clearly the situation is not stable - is meta-stable as WD Hamilton said. We are, at present, in the lag phase when the inertia has allowed modernity to continue after the fundamental basis for its continuation has gone. The uniqueness of this situation, enabled by the truly massive effect the human species has had upon the environment for the selection of the human species during the past few centuries, could explain why the phenomenon has apparently not been previously noticed by biologists - not even the greatest ones. Nonetheless, this is a fundamental attribute of natural selection - http://charltonteaching.blogspot.co.uk/2014/09/the-primacy-of-red-queen-or-what-does.html.)