> The precautionary principle is not an all or nothing concept.  One looks
> at the perceived problem, and then you make a judgment based on previous
> experience etc. as to how severe the perceived problem may be.  And
> then put in place appropriate controls and monitoring.  If the
> monitoring shows a problem, or indeed lack of one, the controls are
> changed.


The proposal above is good risk-balancing but it is not the "precautionary principle" as that phrase is generally used.  The precautionary principle boils down to "do nothing until we can show that it is safe" for some debatable level of 'safe'.  As several have already pointed out, there is no absolute measure of 'safe' nor is there a defined level of proof that is acceptable to all - there will always be some constituency who thinks that more proof or a higher level of safety is needed.  Those lobbying for more research correctly argue that "we don't yet know all the dangers".  
 
There are a number of flaws in that argument.  First, that argument misses the fact that we can never know all the dangers.  Even things that we thought for millenia were perfectly safe (like eating charred meat) have been found to be unsafe to one degree or another.  There is no absolute measure of knowledge that allows us to define when the research is enough to make a decision.  This creates an unbounded situation of moving goal posts.  Any increase in information inevitably leads to either more questions (a fundamental characteristic of scientific advancement) or at least to a renegotiation of how much is enough research.  The threshold for decision-making is not defined ahead of time.
 
Second, the precautionary principle is fundamentally unable to balance known risks against unknown risks.  The precautionary principle views a risk in isolation rather than in the context of other risks and benefits.  It does not account for the risks (known and unknown) of the status-quo practices which will be displaced by the new proposal.  
2a.  Most applications of the precautionary principle begin with the false assumption that the status quo is risk-less - that all risks reside only in the new proposal.  Returning to the charred meat example, it may cause cancer but eating undercooked meat carries the risk of all sorts of parasites and avoiding meat leads to protein deficiencies.  Making decisions based only on the newly perceived risks without regard for the old risks is a logical fallacy.  It is, unfortunately, entirely human.  The phenomenon is called recency bias.
2b.  Even if you can overcome the false assumption of zero risk to the status quo, you get into the cognitive biases of known versus unknown risks.  Known, understood risks are favored even when they are greater in both likelihood and severity than the less-well-understood risks.  That's why we drive routinely yet fear flying despite the dramatically higher fatality rates in automobiles.
 
Third, for any coherent decision about risk, you have to be able to quantify the risk and to evaluate that quantification rationally.  As any casino owner will tell you, humans are notoriously bad at comparing risks.  Even when given hard numbers about risks, humans behave inconsistently with the odds (generally favoring the extremes).  A very few folks with high degrees of training (world-class poker players and some physicists) are able to overcome that bias but even they have difficulty when working out of their usual context.  (There are other cognitive biases that get in the way of good decision-making but these are enough for now.  There are several good texts on the topic.  The laundry list in the Wikipedia article titled "List of biases in judgment and decision making" is not a bad starting point.)
 
Finally, all those cognitive biases lead to sub-optimal solutions.  The precautionary principle demands that unknown risks become known risks before action may be taken, regardless of the relative degree of risk in the status quo.  It leads to the perpetuation of existing risks and costs of the status quo.
 
Neonicotinoids may or may not be significantly harmful to bees.  They may or may not have long-term sub-lethal effects on exposed humans.  They are, however,  unambiguously less toxic to farmers than the organophosphates and pyrethroids that they displaced.  As a society, we have a known (and somewhat quantifiable) risk from organophosphates which now must be balanced against potential and still unknown risks of neonicotinoids.  The precautionary principle does not allow that balancing act.  
 
Mike Rossander

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