A Change From Sensible to Senseless
A civilisation is a shared understanding, and the decline of a civilisation is the decay of this communal understanding from sensible to senseless. A community maintains its understanding by passing its traditions from generation to generation. That is, each generation is charged with rearing its progeny as dutiful (unselfish) citizens who revere the beliefs, hence the manners, customs and laws, which are the tradition of their parents. When this process fails then the community stops maintaining, but starts to corrupt, tradition: that is, it starts to discard or reverse the manners, customs and laws, so they cease to be a duty but become an indulgence.
Corruption Progresses a Generation at a Time
The corruption of tradition progresses a generation at a time, with each succeeding wave of offspring corrupting more tradition, which in turn means showing less restraint and thus less understanding than the last generation.
Becoming Increasingly Silly
As previous restraints are discarded, citizens find themselves less able to resist their private impulses. With a reduced need to control their urges they become more susceptible to the temptations of conceit, vanity, envy, sloth, rage, lust, greed and fear; losing not only the ability to control their actions but their thoughts. Hence with each successive generation the character of the citizens becomes increasing uncontrolled, irresolute and silly.
Or as the ancient Roman poet Horace (BC 65-8) said:
Our parents, worse than our grandparents, gave birth to us who are worse than they,
And we shall in our turn bear offspring still more evil.
Increasingly Vulnerable to Attack
And the result is a community growing ever more demented and impotent — senile; becoming increasingly unable to resist attacks by: