Tom Sutcliffe: I would rather not see my future, thanks

'The best thing about the future", Abraham Lincoln said, "is that it only comes one day at a time". One day, the future was going to place Abe in fatal proximity to John Wilkes Booth, but he did not have to live with that knowledge. And this has long been a universal charity of human existence: that we can't know in detail what lies in store for us.

It is, though, a charity that is steadily being nibbled away by the advances of science. The other day there were reports that scientists had identified a genetic marker which would make it possible to say how quickly a given person would age (and therefore die).

that chimed in my mind with something I'd seen on stage a few days earlier, in Tamsin Oglesby's play about ageing and dementia, Really Old, Like 45, at the National. In one powerful scene Monroe, the head of an institute conducting dubiously ethical research into ways of treating an ageing population, is confronted with the fact that a scan of his own brain has revealed the predictive markers of disease. He doesn't have Alzheimer's yet, but now he knows that he will one day get it – and his research suddenly has a pointedly personal application. The future has made an early down-payment in ways that Lincoln couldn't have imagined.

Oglesby's scene is an extrapolation from a current fact: that you can do a genetic test to find out whether you are likely to succumb to Alzheimer's, well before any symptoms make themselves known. And one of the first things to say about this – and the new discovery about ageing rates – is that they are only refined and special cases of a universal harshness in human existence; that we all know what's coming in the end. Anyone who takes these tests will be able to discern looming and upsetting shapes in the murk that lies ahead. But they haven't suddenly been placed in an entirely different category to the rest of us, who will also lose our faculties and die one day. And this raises the obvious question: if you could buy such a test over the counter at Boots, would you? What exactly would you gain?

The ability to do something about the result is usually offered as the significant factor here. Since there is currently no cure for Alzheimer's and only unverified ways of forestalling it, it's generally regarded that there's not a lot of point in knowing. If you have the gene for early death, on the other hand (or high cholesterol, to take a far more common example), you can take some meaningful steps to bend your fate in a more favourable direction. And certainly there will be people for whom nothing less than a knock on the door by a bloke with a scythe will be necessary to get them to change their habits.

But even in these cases I wonder whether the trade-off makes sense. The problem is that you will have exchanged a common (and therefore more bearable) uncertainty for an entirely delusive certainty that threatens to overshadow the life you have left. And you may not be able to rid yourself of the feeling that you are making sacrifices now (these things usually do involve sacrifice, otherwise everyone would be doing them already) for a dubious return in the distant future.

This isn't to argue that it doesn't make sense for us to look after our health, incidentally, only that it might be wiser to do so because of what it delivers today and tomorrow – not in 20 years' time. After all, John Wilkes Booth, or a bus, may get us long before we get to where science – in entirely good faith – told us we were going. The unavoidable thing about life – and possibly the best thing, too – is that you can only live it a day at a time.

This poster was asking for trouble

Political posters have always been a honeypot for graffiti artists. Indeed you could argue that if one doesn't get defaced at all, anywhere, it hasn't done its job properly.

But there is a fine line between provoking a response and the glorious satirical caption-competition which the Conservatives have unleashed with their "Year for Change" campaign. Has a general election poster ever provided a more inviting blank canvas for contrarians and mischief-makers than this one?

The funniest spoofs are to be found online, at mydavidcameron.com . I'm fond of the one that replaces the original copyline with "They took away my tie and shoelaces. But they forgot to lock the cell". And there's also a jolly number in which the slogan "We can't go on like this. Get me an airbrush!" is accompanied by a notably twerpish portrait of Cameron.

Mydavidcameron.com is running a competition to find the best one and the current leader in the voting when I looked is an unaltered image with the line, "Some of my best friends are poor". My favourite, though, is the one that substitutes Cameron with Tim Nice-But-Dim from The Fast Show, which is only behind by a narrow margin. Remember, people died for your right to vote. Don't waste it.

Discrimination laws can go too far

Listening to Sir Mota Singh defend the right of a north London schoolboy to carry a five-inch knife to school, on the grounds that wearing the Kirpan is an article of faith, I was struck by his use of the word "discrimination".

This was intended to frame the matter as one of religious liberty rather than school discipline and safety. And behind it was the assumption that all forms of discrimination are indefensible. I don't think this one is, though. I approve of the idea that my children's schools will discriminate against pupils who carry knives – however heartfelt their reasons for doing so and however unlikely it is that they would use them. If the Kirpan truly is a ceremonial object – constructed in such a way that it can't be used to stab – then there shouldn't be a problem.

But if it's a knife it shouldn't be allowed in. The mistake is to think that the enforcement of such a rule (and the tactful insistence on a sensible compromise by religious authorities) must always represent a humiliating triumph of secularism over religious faith. It doesn't. It simply asserts that there are places where the values of a civil, shared society – indifferent to individual convictions – come first. Everyone has an interest in that – and minority religions in particular will find it a far better protection of their liberties than a prickly insistence on being exempted from rules that discriminate against nobody because everyone is included.