The world is mere change, and this life, opinion.

What happened? It took Gibbon six volumes to describe the decline and fall of the Roman Empire, so I shan't embark on that. But thinking about this almost incredible episode does tell one something about the nature of civilisation. It shows that however complex and solid it seems, it is actually quite fragile. It can be destroyed. 

What are its enemies?

Well, first of all fear — fear of war, fear of invasion, fear of plague and famine, that make it simply not worthwhile constructing things, or planting trees or even planning next year's crops. And fear of the supernatural, which means that you daren't question anything or change anything. The late antique world was full of meaningless rituals, mystery religions, that destroyed self-confidence. And then exhaustion, the feeling of hopelessness which can overtake people even with a high degree of material prosperity. 

There is a poem by the modern Greek poet, Cavafy, in which he imagines the people of an antique town like Alexandria waiting every day for the barbarians to come and sack the city. Finally the barbarians move off somewhere else and the city is saved; but the people are disappointed — it would have been better than nothing. Of course, civilisation requires a modicum of material prosperity— 

What civilization needs:

confidence in the society in which one lives, belief in its philosophy, belief in its laws, and confidence in one's own mental powers. The way in which the stones of the Pont du Gard are laid is not only a triumph of technical skill, but shows a vigorous belief in law and discipline. Vigour, energy, vitality: all the civilisations— or civilising epochs— have had a weight of energy behind them. People sometimes think that civilisation consists in fine sensibilities and good conversations and all that. These can be among the agreeable results of civilisation, but they are not what make a civilisation, and a society can have these amenities and yet be dead and rigid.

The word Familia did not originally signify the ideal of our modern philistine, which is a compound of sentimentality and domestic discord. Among the Romans, in the beginning, it did not even refer to the married couple and their children, but to the slaves alone. Famulus means a household slave and familia signifies the totality of slaves belonging to one individual. The expression was invented by the romans to describe a new social organism, the head of which had under him wife and children and a number of slaves, under Roman paternal power, with power of life and death over them all.