You're a concerned, ethical citizen. You care about the impact of your shopping on the environment and your health. So which tomatoes should you buy: organic or conventional?

Until now, most people have assumed that organic is the better choice, but it is becoming increasingly obvious that the organic/ conventional distinction fails to draw the line between good and bad - agriculturally, environmentally and nutritionally.

This weekend, the former head of the Food Standards Agency, Lord Krebs, repeated his claim that organic food has no particular health benefits. An FSA spokesman backed him up, saying: "The weight of scientific evidence does not support claims that organic food is more nutritious or safer than conventionally produced food."

A report earlier this year for the Department for the Environment, Food and Rural Affairs also concluded that many organic foods grown in the UK are less energy efficient and more polluting than conventional equivalents.

So the organic tomatoes may have had more of an environmental impact and be no better for you than the conventional ones. Still, supporters of organics have something on their side more powerful than fact: the belief that "natural" is best.

But don't ask hard questions about what is natural and why it should be superior. For example, organic farmers can use the bacterium bacillus thuringiensis for pest control. But the idea that this is intrinsically safer than applying chemicals doesn't stand up to rational scrutiny. Wahat matters is that what farmers use is safe, not whether it is naturally occurring. Arsenic is natural, but I wouldn't sprinkle that on tomatoes.

Another organically-approved pesticide has been shown to cause the symptoms of Parkinson's disease when injected into rats. I'm not suggesting this means it is unsafe, but why is it that the precautionary principle only seems to apply to synthetic pesticides?

At a time when the virtues of organic food are being questioned as never before, you might think its supporters would at least want to maintain a clear line about what organic means. Instead, they're trying to muddy the waters even more.

The UK's leading organic certification organisation, the Soil Association, wants to withhold the organic label from some air-freighted produce. Its intentions are noble: we need to reduce carbon emissions to fight climate change. But what has this got to do with organic production? My apple doesn't stop being organic because I take it on a plane to Kuala Lumpur.

The proposals don't even make sense when judged against the objective of reducing carbon in the atmosphere. The Soil Association intends to allow air-freighted produce to be certified as organic if it meets "ethical trading" standards. That would mean that an organic farmer in Ayrshire could produce more CO2 by growing his tomatoes in heated greenhouses than a farmer in Kenya, even allowing for the air freight's emissions, yet he would be allowed to call his produce organic and the Kenyan would not.

Intellectually, then, the very idea of "organic" is a mess. It makes health and environmental claims that cannot be backed up; it distinguishes between acceptable and unacceptable pesticides and fertilisers on a crude test of how "natural" they are, rather than on how safe they might be; and it is now making the size of carbon footprint part of the criteria for organics, even though there is no precedent for this and the rules it is proposing make no sense.

So is there anything coherent keeping the organic movement together? Look at what it actually supports and opposes, and a clear vision does emerge: organics is really about resistance to modern technology. Old pesticides, heated greenhouses and trucks are fine, but new chemicals and airplanes are not. The rules about what counts as "organic" are rigged to support a world view in which it is a matter of faith that what is old and traditional is better. No wonder Prince Charles is such a fan.

Consumers are wising up to this, however. People are realising that organic tomatoes grown in heated greenhouses and sold in plastic containers are not necessarily better than conventional ones trucked in from the Mediterranean to be sold loose. We are all gradually learning to make purchasing decisions on the basis of a more sophisticated understanding of what good, ethical food and farming requires.

The illusion that organic is always the moral, healthy choice is starting to crumble. It's time we grew up and accepted that good, sustainable, ethical food is not guaranteed just because it carries an organic label.