Please take your left hand and place it in a bucket of cold water. Thank you. Now take your right hand and place it in the hottest water you can bear.

Done that? Right, take both hands and place them in a bowl of lukewarm water. And, with your fists immersed, start thinking about Gordon Brown and David Cameron.

I am going to put to you a perfectly simple political idea. It is most commonly said of Labour and the Conservatives that no one really knows what they stand for. I believe that the first party to make clear to voters what it really believes in will... lose.

OK. You can take your hands out of the lukewarm water now.

The point of that rather odd, and wet, exercise was to illustrate how we perceive things. When placed in the lukewarm water, the hand that was in the cold water will feel warm, while the right hand, having been in the hot water will, in the same bowl of lukewarm water, feel cold. This is what is known as the contrast principle. The feeling in your hand is not set by the absolute temperature of the lukewarm water, but by the contrast between its temperature and the temperature of the first bowl.

This idea crops up a good deal in the literature on social psychology and persuasion. When making a choice, the only way that you can make sense of what you are being offered is by comparison with similar offers. How else would you know, even roughly, what price to pay, or what qualities to look for?

In his book Predictably Irrational, Professor Dan Ariely, of Massachusetts Institute of Technology, provides a host of examples of the principle in practice. The Economist magazine, for instance, offers readers a one-year subscription to Economist.com for $59 and a one-year subscription to the print edition for $125. It also offers, in the same web advertisement, a joint print and internet subscription for $125.

Why this rather odd set of choices? People choose things using the contrast principle. When assessing the print-versus-internet option, they might choose the cheaper internet-only option. But given all three, they might view the combined, more expensive, offer as a genuine bargain - you get the internet free! Professor Ariely conducted tests and found that this was precisely how consumers reacted.

Now let's look at how voters understand the offers made by political parties. They cannot make much sense of the absolute positions taken by the parties, so instead they employ the contrast principle. They understand what parties stand for by comparing them with each other.

This is an important reason why voters and pundits commonly complain that the parties don't really stand for anything. This complaint isn't quite right. Of course there are large areas of ambiguity, and a good deal of fudge, but both parties have reasonably clear stances on any number of things. It is just that while their positions are somewhat different, they are similar enough for voters to have difficulty understanding the differences, much of which lie in the detail. If voters can't contrast, they can't understand.

So what voters and pundits really mean when they say that parties don't really stand for anything is that the things they stand for are too similar. And the only way parties can respond is by making themselves different from the other guys. And that is when they start losing.

After the 2005 election David Cameron deliberately shifted his party closer to the position of the Government. This was the same move that Tony Blair had made before the 1997 election. In both cases they concluded that a broadly moderate, centre position was superior as an electoral strategy and a governing philosophy. They sacrificed definition by reducing the differences with their opponents, but regarded the trade-off as worthwhile. And they were surely right.

Between 1997 and 2005, the Tories had made a different calculation. Anxious to seem as if they were standing for something, they strove always to be distinctive. The moment that the Conservatives succeeded in showing that they were different to Mr Blair, the contrast principle kicked in, and voters understood their position for the first time. And then they went out and chose Mr Blair rather than them.

During this period Gordon Brown developed his idea of fighting elections by establishing dividing lines. Of course he did, because he knew that Labour would, electorally at least, always be on the right side of them. The Tories were willing accomplices in this strategy, even though it led to their own electoral defeats.

The problem that Mr Brown now has is that the Tories appear to have stopped playing that game. The only way for the Prime Minister to establish dividing lines now is by shifting from the centre position that he inherited from Mr Blair; the only way that he can establish his vision is by creating differences with the Tories that put him on the wrong side of the line. Stay where he is and people wonder what was the point of him becoming Prime Minister. Move to the right and the electorate is simply confused. Move to the left and he has abandoned both the correct and popular position. It's not much of a choice.

His best hope is that Mr Cameron loses his nerve. As the election gets closer, so the pressure for the Conservatives to define themselves will get greater. The Tory leader will find that producing detailed policy proposals will not satisfy those who call for greater clarity. He will find that the appetite will not be sated by big tough decisions on controversial issues. Nor will set-piece platform addresses do the trick. The only way of satisfying the demand that he stands for something would be to leave the centre ground. He is better off leaving that demand unsatisfied.

Mr Brown must pray that Mr Cameron cannot resist being seen as distinctive. My guess is that, in this, the Prime Minister will be disappointed. I think the Tories have learnt that lesson.

daniel.finkelstein@thetimes.co.uk

Times Recommends

• Tear down this wall! And save the planet
• Double click if you want the bin emptied
• Have you got permission to read this?