At the heart of Haiti's humanitarian crisis is an age old question for many religious people - how can God allow such terrible things to happen? Philosopher David Bain examines the arguments.

Evil has always been a thorn in the side of those - of whatever faith - who believe in an all-knowing, all-powerful, all-good God.

As the philosopher David Hume (echoing Epicurus) put it in 1776: "Is God willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Whence then is evil?"

Faced with this question, Archbishop of York John Sentamu said he had "nothing to say to make sense of this horror", while another clergyman, Canon Giles Fraser, preferred to respond "not with clever argument but with prayer".

Perhaps their stance is understandable. The Old Testament is also not clear to the layman on such matters. When Job complains about the injuries God has allowed him to suffer, and claims "they are tricked that trusted", God says nothing to rebut the charges.

Less reticent is the American evangelist Pat Robertson. He has suggested Haiti has been cursed ever since the population swore a pact with the Devil to gain their freedom from the French at the beginning of the 19th Century. Robertson's claim will strike many as ludicrous, if not offensive.

And even were it true, it wouldn't obviously meet the challenge.

Why would a loving deity allow such a pact to seem necessary? Why wouldn't he have freed the Haitians from slavery himself, or prevented them from being enslaved in the first place? And why, in particular, would he punish today's Haitians for something their forbears putatively did more than two centuries before?

So what should believers say? To make progress, we might distinguish two kinds of evil:

• the awful things people do, such as murder, and the awful things that just happen, such as earthquakes.

But there's a counter-argument. Thoroughly good people aren't robots, so why couldn't God have created only people like them, people who quite freely live good lives?St Augustine, author CS Lewis and others have argued God allows our bad actions since preventing them would undermine our free will, the value of which outweighs its ill effects.

However that debate turns out, it's quite unclear how free will is supposed to explain the other kind of evil - the death and suffering of the victims of natural disasters.

Perhaps it would if all the victims - even the newborn - were so bad that they deserved their agonising deaths, but it's impossible to believe that is the case.

Or perhaps free will would be relevant if human negligence always played a role. There will be some who say the scale of the tragedy in natural disasters is partly attributable to humans. The world has the choice to help its poorer parts build earthquake-resistant structures and tsunami warning systems.