How did he do it? First of all, with the help of an outstanding teacher and librarian named Alcuin of York, he collected books and had them copied. People don’t always realise that only three or four antique manuscripts of the Latin authors are still in existence: our whole knowledge of ancient literature is due to the collecting and copying that began under Charlemagne, and almost any classical text that survived until the eighth century has survived till today.

Charlemagne is the first great man of action to emerge from the darkness since the collapse of the Roman world. He became a subject of myth and legend. A magnificent reliquary in Aix-la-Chapelle, made about five hundred years after his death to hold a piece of his skull, expresses what the High Middle Ages felt about him in terms that he himself would have appreciated – gold and jewels. But the real man, about whom we know quite a lot from a contemporary biographer, wasn’t so far from that myth. He was a commanding figure, over six feet tall, with piercing blue eyes – only he had a small squeaky voice and a walrus moustache instead of a beard. He was a tireless administrator. The lands he conquered – Bavaria, Saxony, Lombardy – were organised a good deal beyond the capacities of a semi-barbarous age. His empire was an artificial construction and didn’t survive him. But the old idea that he saved civilisation isn’t so far wrong, because it was through him that the Atlantic world re-established contact with the ancient culture of the Mediterranean world. There were great disorders after his death, but no more skin of our teeth. Civilisation had come through.

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 – enough to provide a little leisure. But, far more, it requires confidence – 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 great civilisations – or civilising epochs – have had a weight of energy behind them. People sometimes think that civilisation consists in fine sensibilities and good conversation 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 an

It means that at certain epochs man has felt conscious of something about himself – body and spirit – which was outside the day-to-day struggle for existence and the night-to-night struggle with fear; and he has felt the need to develop these qualities of thought and feeling so that they might approach as nearly as possible to an ideal of perfection – reason, justice, physical beauty, all of them in equilibrium. He has managed to satisfy this need in various ways – through myths, through dance and song, through systems of philosophy and through the order that he has imposed on the visible world. The children of his imagination are also the expressions of an ideal.

Everybody who writes about Blake begins by saying that he was a visionary. It is a vague term. All artists, even the most realistic, start with some kind of vision - that is what leads them to select what they need from the infinite diversity of appearances.

The Vikings were quartering the world; they set out from a base, and with unbelievable courage and ingenuity, they reached as far as Persia by the Volga and the Caspian Sea. Then they returned home with all their loot in these open ships, including coins from Samarkand and even a Chinese Buddha. The sheer technical skill of their journeys was a new achievement, and their spirit did contribute something very important to the Western world, because in the end, it was the spirit of Columbus.

At some time in the ninth century one could have looked down the Seine and seen the prow of a Viking ship coming up the river. Looked at today in the British Museum it is a powerful work of art; but to the mother of a family trying to settle down in her little hut, it would have seemed less agreeable – as menacing to her civilisation as the periscope of a nuclear submarine.