Here's a link we have to break: the association, both real and imagined, between environmentalism and alternative medicine. Prince Charles springs to mind.

Environmentalism is, or should be, a movement led by scientific findings. I see the role of environmentalists as being to explore and explain the implications of what the science – whether on climate change, habitat loss, biodiversity, fisheries, pollution or resource depletion — is saying, and how this should translate into public policy. We should try at all times to be rigorous. And we should kill our darlings – our enthusiasm for solar panels, for example, or our rigid opposition to nuclear power — if the facts demand it.

This doesn't mean that we have to be motivated by the science. My environmentalism arises from both a deep love of the natural world and a strong sense of the injustices done to vulnerable people: it's an emotional impulse, in other words.

But we must at all times be informed by it. There is no room for wishful thinking. What is the point of dedicating your life to campaigning, only to discover that you have wasted it because the facts don't support you? There is a subtle difference between sticking to your principles – justice for the living and the unborn, the defence of a healthy biosphere, for example – and sticking to your beliefs. We must doubt everything, question everything, believe nothing until it has been demonstrated, and even then subject it to continued scepticism and enquiry. Above all, we must never allow ourselves to imagine that we are finally and definitively right about anything.

The great majority of alternative medicine, by contrast, relies to some degree on wishful thinking. Indeed, as the admirable Ben Goldacre keeps showing, such efficacy as it might possess is largely due to a deep and subtle variety of wishful thinking known as the placebo effect. Yet despite the fact that these two disciplines are, or should be, chalk and cheese, I know plenty of people who subscribe to both: who call themselves environmentalists but who use, promote or practice alternative medicine.

The most famous example is Prince Charles, who has just been attacked by Edzard Ernst, professor of alternative medicine at Exeter university, for promoting "outright quackery" and engaging in "make-believe and superstition" by selling something called Duchy Herbals Detox Tincture. The facts, as Rose Shapiro's excellent book, Suckers, explains, appear to favour Professor Ernst.

Prince Charles is a complex character. Sometimes he makes a good deal of sense. What he said about the Mumbai slums last month was daring but, I think, broadly right. In Santiago on Monday, he spoke persuasively about the link between poverty and climate change.

At other times he talks nonsense. He put the campaign against GM foods on the map but he also wrecked his case, by waffling on about God and the natural order rather than the genuine threat to global food security caused by the corporate control of the food chain.

Either way, I believe that he should decide whether he is a prince or a pundit. Because the prominence of the heir to the throne's political views results solely from his inherited position, rather than from his persuasiveness or eloquence, his outspokenness looks to me like an abuse of power.

Progressives who support his right to speak out without abdicating should consider how they would feel if he shared the opinions of his maternal grandmother who – as Francis Wheen explained in the Guardian – was an enthusiast for appeasement, an anti-Semitic xenophobe and a sympathiser with apartheid.

Anyway, the point is that he's not alone.

Perhaps because using alternative medicine means, for the most part, boycotting the business of the biggest and most brutal pharmaceutical companies, perhaps because the alternative medicines industry has done much to promote itself as "natural" I know plenty of greens who can explain in some detail how radiative forcing works, yet who use homeopathy or won't expose their children to MMR injections.

That's up to them. But the association in the public mind does us nothing but harm. Those of us who do not believe there is a link between a movement based on science and a business based largely on nonsense should say so loudly and clearly.

Monbiot.com