As Chris Titley is on holiday today's column is by Stephen Lewis.

A WISE man once said the reason so many people needed a God to believe in was because they couldn't bear the responsibility of being grown up. It's a difficult thing, growing up. Not the rites of passage stuff - being a teenager, learning about drugs, booze, sex, relationships. That is hard, but it's a phase we go through.

The really hard thing is accepting you're an adult and have to take responsibility for your own actions.

Many of us never manage it. You only have to look at the short-term greed on display in the row over fuel duty, or the way US presidential hopeful George 'Dubya' Bush was able to garner millions of votes by the simple expedient of pledging to cut taxes and hang the consequences.

By and large, we'll grab what we can for our own short-term benefit - and if things go wrong there will always be someone around to blame. If it's not God, it'll be the council, the law or, best of all, Tony Blair.

But we live in an increasingly grown-up world: a world where the possible consequences of our actions grow ever more catastrophic.

One of the most frightening things about modern science is the way it all too often outstrips our ability to know what to do with it.

Nuclear power, cloning, GM crops - even the humble internal combustion engine - they've all, in their way and in their day, posed ethical dilemmas about how far we should allow science to go that we're just not equipped to deal with.

The case of the Siamese twins Jodie and Mary is a classic example. In their short lives so far the two tiny girls - each a unique human being, even though they share a fused spine and joined abdomen - have ignited a debate to which there is no right answer.

Is it right that one life should be sacrificed - a life that is doomed to end anyway - in order that another should continue?

Put so starkly it may seem the answer must be yes. And yet the physician's prime function is to preserve life. The surgeons who this week performed the complex surgery needed to separate the twins knew that by doing so, they were ending the life of one, Mary - and that they were acting against the express wishes of the twins' parents, who felt nature should be allowed to take its course.

Just how difficult the moral and ethical issues involved are - and how ill-equipped our legal system is to handle them - was demonstrated by the actions of High Court judge Lord Justice Ward.

The judge - one of three who jointly decided that Mary was 'unviable' and 'designated for death' and that it was right the operation should go ahead so her sister Jodie could be given a chance of life - spoke movingly outside court of the 'excruciating' dilemma posed by the case.

Right or wrong, the deed now has been done. All will surely hope and pray that Jodie will survive and thrive.

But the point about the heart-wrenching case of the two helpless twins is that the dilemma posed by their condition is one that none of us - not the parents, not the doctors, not the judges who ultimately shouldered the burden of responsibility for the decision - would have faced but for the relentless advance of science.

As we march into the 21st century, we will face many more such dilemmas. Many of them will have ramifications far beyond the life of a single child, however poignant. We each must share in the responsibility of making sure the decisions we take are the right ones.

We must start thinking about long-term solutions to the problems - pollution, global warming, overpopulation - science has left us with. We must learn to grow up.