A slightly different approach is where the uncertainty involves what economists like to call "fat tails". This is something that has latterly preoccupied the eminent Harvard economist, Martin Weitzman. It arises where there are a range of possible outcomes, including an extreme outcome which, however unlikely, cannot be ruled out, and which — if it occurred — would be so catastrophic that, it is argued, there is no limit to what we should spend to reduce the risk. Professor Weitzman has written about this in the context of catastrophic global warming. But there are two great difficulties with this approach. The first is that there is not the remotest chance of securing global agreement on the proposed course of action. The second, and more fundamentally, is that, even if this could be overcome, there is a whole range of possible catastrophes that cannot be ruled out altogether.

Thus, in addition to catastrophic global warming, there are — to name but a few — the coming of a new ice age (as the distinguished Cambridge scientist, Sir Fred Hoyle, warned in his book Ice), a nuclear holocaust, the mother and father of all pandemics (or plagues, as they used to be known) or a large asteroid hitting the planet.

To which the current President of the Royal Society, Martin Rees, in his book Our Final Century? has added the risk of nanotechnology running amok and what he terms "bioterror or bioerror" from developments in biotechnology.

The problem is obvious: if we were to devote unlimited resources to guard against every conceivable "fat tail" risk, there would be nothing left for us to live on.

We have to use our common sense, based on our best estimate of probabilities — and, in particular, time scales, which Weitzman appears to ignore; since a threat of what might happen in a thousand years time is clearly less pressing than one that might occur in ten years time.

This, indeed, is what the recently published UK strategic defence review sought to do. But one thing is quite clear: the so-called precautionary principle is merely a term of advocacy, of no substantive meaning or use whatever.