I’m not sure that King Canute was being an optimist when he commanded the tide not to rise and dampen his feet: in fact, he was being a pessimist and proving to his people that he was not infallible, and that the tide would not obey his command. He has become legendary because of that bit of wisdom.

We could do with that wisdom today: to know the limits our power and when to exercise our optimism.

We complain about immigration, and our legislators bulldoze half-empty refugee camps; but the tide of migration from south to north, from poor to rich grows rapidly. Now the systems in Greece, Italy and Spain are under strain, and a wall between the USA and Mexico won’t stop the tide.

Why? Do people who live in nice warm countries really want to live in cold damp, overcrowded, foreign lands such as England? Probably not: the reasons are conflict, bad governance and lack of work and food: circumstances that make it impossible for people to better themselves.

But at the root, what is driving people to go through horrendous, life-threatening hardship, deserting all that is familiar, is over-population: the more people there are, the more competition there is for resources. The general result is poverty, starvation, environmental destruction (eg chop trees down around Manilla and the city floods) and conflict at local, regional and national levels. And as each person on this Earth adds to the environmental burden, especially whilst our behaviour is so out of control and we pay so little heed to what is sustainable, the drives to migrate are only increasing.

The tide is coming in and, whilst there is the pressure of growing population behind it, we are as powerless to stop south-north migration and the conflict that will result as old King Canute was to stop the sea. But there are two differences: the first is that, unlike Canute, we are glad-eyed optimists who seem to believe that things will sort themselves out, or some technology will come along to do it for us; and second that Canute could do nothing about the sea and the moon, whereas humanity could tackle the problem of exploding population.

The Optimum Population Trust is a UK think tank that is trying to find some answers. www.optimumpopulation.org