I have a proposition to put to you. Our society, and even our wonderful party, are too in love with the concept of evidence.
Perhaps this surprises you. Maybe you're thinking: 'If only!' If only enough attention were paid to the massive evidence that dangerous climate change is happening, and that it's human- triggered. Or: If only the epidemiological evidence marshalled by Wilkinson and Pickett - that more inequality makes society worse in almost every conceivable way - were acted upon.
But actually, even in cases like these, I think that my proposition is still true. Take human-triggered climate change. Yes, the evidence is strong, but a 'sceptic' can always ask for more/better evidence, and thus delay action. There is something stronger than evidence: the concept of precaution.
A sceptic, unconvinced by climate- models, ought to be more cautious than the rest of us about bunging unprecedented amounts of potential pollutants into the atmosphere! Any uncertainty over the evidence increases our exposure to risk, our fragility.
The climate sceptics exploit any scientific uncertainty to seek to undermine our confidence in the evidence at our disposal. So far as it goes, this move is correct. But our exposure to risk is higher the greater the uncertainty in the science. Uncertainty undermines evidence, but it doesn't undermine the need for precaution: it underscores it! Remember how high the stakes are.
Think back to the great precedent for the climate issue: the issue of smoking and cancer. For decades, tobacco companies prevaricated against action being taken to stop the epidemic of lung cancer. How? They demanded incontrovertible evidence that smoking caused cancer, and they claimed that until we had such evidence there was nothing to be said against smoking, health-wise. They deliberately evaded the employment of the precautionary principle, which would have warned that, in the absence of such evidence, it was still unsafe to pump your lungs full of smoke, day in day out, in a manner without natural precedent.
We ought to have relied more on precaution and less on evidence in relation to the smoking-cancer connection. The same goes for climate.
And also for inequality: Wilkinson and Pickett are merely confirming what we all already ought to have known anyway: that it's reckless to raise inequality to unprecedented levels, and so to endanger society itself - for how can one have a society at all, when levels of trust and of co-mingling are ever-decreasing?
The same goes for advertising targeted at children. It's outrageous to demand evidence that dumping potential toxins into the mental environment actually is dangerous; we just need to exercise precautious care with regard to our children's fragile, malleable minds.
And also for geo-engineering: There's no evidence at all that geo-engineering does any harm, because (thankfully!) it hasn't been carried out yet. In this case, we must be 'precautious', for, by the time any evidence was in, it would be too late.
The same goes for genetically-modified (GM) crops. There is little evidence of harm, to date, from GM, but evidence is the wrong place to look: one ought to focus on the generation of new uncertainties and of untold exposures to grave risk that is an inevitable consequence of taking genes from fish and putting them into tomatoes, or on creating 'terminator' genes, et cetera... The absence of evidence that GM is harmful must not be confused with evidence of absence of potential harm from GM. We lack the latter, and thus we are direly exposed to the risk of what my philosophical colleague Nassim Taleb calls a 'black swan' event: a massive known or even unknown unknown.
So: let's end our love affair with evidence. Yes, being 'evidence-based' is usually (though not always!) better than nothing. But there's nearly always something better still: being precautious. (And, what's more, being precautious makes it easier to win, and quicker.)
This is the unique contribution that Greens can offer to public debate.
Rupert Read is a philosopher of science, and chairs Green House: www.greenhousethinktank.org.He is hoping to stand in Cambridge at the General Election