More Reasons Not To Use The Precautionary Principle: Update

A member of Congress in repose?
A member of Congress in repose?

All probability is conditional and we are always interested in some proposition, call it X. We want to know “the probability of X”. Well, there is none: not ever. There is no unconditional “probability” of anything.

There is, however, Pr(X | E), where E is some evidence (data, observations, premises, surmises, whatever). Change the evidence, change the probability.

In the precautionary principle, X is some disaster or undesirable event. Now it is easy to supply some E to deduce a probability of X. If there was widespread agreement on this E, then there would be agreement on the probability of X. The opposite case is also true. In global warming, there is great disagreement about evidence, usually because lovers of models chose to forget their creations’ flaws. Love is blind.

But the precautionary principle doesn’t quite work that way in practice. Usually the X is dire and the E is missing except to say E_c = “X is possible”, which is another way of saying X is contingent. All contingent things are logically possible; i.e. they are not impossible. With E_c, officially 0 < Pr(X | E_c) < 1, which tells us almost nothing except that X is not logically impossible and that X is not logically necessary. Weak evidence indeed.

Notice very carefully that Pr(X | E_c) is not 0.5, nor any other single number. You can’t use this interval to argue the probability “may be likely”. This is a false reading. The probability is the interval.

Another point of stress: if we knew or agreed upon decent E such that Pr(X|E) is greater than some decision threshold, then we do not need to use the precautionary principle. We’d have Pr(X|E) and we can use the some regular form of decision making. The precautionary principle is only invoked when such evidence is missing. Indeed, it is used to supply the missing evidence. The argument is that we don’t know E so we don’t know the probability but we do know E_c, thus X could happen, therefore X is sufficiently likely, thus we ought to do something. This is obviously a fallacy.

Or the precautionary principle is used when evidence exists which shows the chance of X is very low indeed. Say E_r (for realized evidence) then Pr(X|E_R) = ε > 0, but only just. It’s then said that because this probability is greater than 0, this is sufficient if the doom in X is disastrous enough. That’s why yesterday at The Stream I illustrated the precautionary principle with a hostile alien invasion.

An invasion is a contingent event, so it’s logically possible. There exists lots of (non-quantitative) evidence that this chance is near-zero low, such as the vast distances of space and so forth. But it could happen! And, like I said, if it did, nothing short of the Apocalypse would be as bad. Thus, according to the precautionary principle, we should—even must!—act to stop it.

Yet alien invasions are only the start of contingent doomsday events that might destroy us all. Rocks from space, viral mutations, planetary plagues, black holes plunging into the ocean, serial volcanic eruptions, the core of the Earth spinning out of control, Hillary presidency, rapacious nanobots, rogue humanoid robots, and on and on. Because each is possible and each would destroy mankind, no amount of protection is too little.

And then there are the troubles I mentioned yesterday, the precautionary principle applied to itself. If we can’t agree on the evidence such that we can say something about the probability of X, then the effects of protecting against X by manipulating the causes or possible causes of X are also likely unknown, and just as likely as hazardous, or perhaps even more hazardous, than X itself.

The solution is boring. Return to the hard work of amassing evidence such that we can agree on the evidence and compute reasonable probabilities. Tough, grueling, time-consuming labor.

Or you can run around like an addled fool and call your detractors “Deniers!” or “Troglodytes!”


Update Another Twitter interaction.

I hope readers can see that.

If a “black swan” is defined as X = “an event which we know nothing about”, then we cannot find evidence E probative of it. There is no probability. Notice that that “nothing” is a very strong word. Make sure you get this.

What I have seen is that some define “black swans” as events which we can characterize at least partly. For instance, X = “Destruction of the human race” (or, in vulgar terms, X = “Loss of all capital”) . No idea in X how the event comes about, thus finding evidence probative of it is a problem. Like I said above, we can go on endlessly positing different ways the world can end. All these can form E, which, taken together, make X all but certain. Ponder this.

But because we packed everything into E, and formed a frightening probability of X, we have learned nothing. Which of the elements of E should we protect against—if it’s even possible? We can’t say “all of them”, because this is silly.


  1. Can’t be a member of Congress in repose–any creature that made it here from outer space is either smart enough to know better than to try and assimilate into the human race or strong enough to just wipe us out. No, sadly, the losers we elect are 100% human. (David Icke takes exception to this view, I should note. He knows they are reptilian. Real alien reptilians, not the “snakes in the grass” term people may use to describe them.)

    As my psychology professor said “Anything is possible. We are only interested in what is probable and what is actually happening. Possible is for philosophical discussions and science fiction.”

    The Precautionary Principle serves the same purpose as the DNA bullying does–to get people to do things that an otherwise rational human would not do. It also forms the basis of the livelihood of personal injury lawyers.

    There was a somewhat illustrative reaction to the CERN collider coming on line–it MIGHT open a black hole, or find antimatter and cause the destruction of the earth, etc.

    Come on. Chicken Little has always been in the top ten of heroes of the frightened, huddled masses. What can I say…..

  2. steve davison

    From its expression I’d suggest it died painfully. Sudden insertion of a hockey stick somewhere sensitive perhaps?

  3. Dean Ericson

    Why wasn’t the Precautionary Principle invoked against redefining marriage? Or against transgenderism? Or against making equality the organizing principle of society or loosening constitutional bounds to create the world’s biggest fascist/communist police state or any of the other pernicious novelties of the revolution? Ah, I know — because the PP is revolutionary sophistry in support of the revolution!?

    Here’s a reactionary PP: Because the potential danger to society posed by out-of-control revolutionaries is cataclysmic-hockeystick-apocalyptic-armageddon the wise precaution would be to immediately imprison all leftists. Works for me.

  4. MattS


    “Come on. Chicken Little has always been in the top ten of heroes of the frightened, huddled masses. What can I say…..”

    But Chicken Little was right. The sky is falling. If it wasn’t falling it would float off into space and that would be really bad. 🙂

  5. Gary

    “The Precautionary Principle serves the same purpose as the DNA bullying does–to get people to do things that an otherwise rational human would not do. It also forms the basis of the livelihood of personal injury lawyers.”
    Sheri, +1. That one goes on my list of favorite modern quotes and aphorisms.

    Briggs, for clarification, would you give the definition of E_c ? I’m guessing it stands for contingent evidence, but exactly what does that mean. What depends on what?

  6. Briggs


    Exactly: E_c = “X is contingent”.

  7. John B()

    Hillary presidency … is possible and … would destroy mankind, no amount of protection is too little.

    Get Arnold to go back in time and kill Hillary’s mother…

    MattS: Clever and correct

    The earth is warming – otherwise the heat would eventually dissipate and it’d be a ball of ice.

    Sheri: good response to JMJ last post (I was going to try)

    It’s why I resent Pascal’s Wager … it make’s a mockery of faith and the evidence of God … God can be known

  8. Corollary to Precautionary Principle**:

    “Just because you’re paranoid doesn’t mean everyone is not out to get you!”

    ** Source: my father

  9. Matt: Chicken Little had the same problem global warming people have–the sky is falling but the rate is so slow it’s not a threat.

    Gary: Thank you.

    John B(): Can we do that? Will it take 4 movies and a couple of TV series to accomplish? I’m all in if we can find a way to do it!

    John Z.: Correct. It doesn’t mean everyone is not out to get you. Occasionally someone is, but it’s not the way to bet. ; )

  10. bernie1815

    Judith Curry has an interesting post that provides more details about the issues afflicting the application of the precautionary principle to climate change.

  11. Sander van der Wal

    Because everybody dies, the only interesting probability is the change that Asteroids will invade Earth *before* one dies. After all, nobody expects dead people to help them. Dead people don’t make dinner, drive you to the hospital, enlist in the Army, or pay taxes.

    Why then should dead people help against things that are so rare that nobody has any idea what is required? Are dead people famous for their knowledge in all things unknown to the living? Known to communicate said knowledge in an obvious and well-understood manner? Has there been some sort of scientific breakthrough that has been snowed under in the Climate Change brouhaha?

  12. DL

    The precautionary principle is illogical because it requires (deductive) proof of being not guilty and it rejects the fundamental inductive principle. This reminds me of Popperian principle of falsification.

    As James Franklin writes in his book “What science knows”:
    “What was his [Popper] answer to the question, “If a theory has survived falsification (after rigorous testing), is it any more probable (credible, rational, believable …) than it was before?” His answer was “no,” as it had to be, given his denial of any probabilistic logic that would support the concept of an increase in probability on given evidence. In Popper’s view, if a theory has survived, it is simply a survivor—that is all that can be said about it. It is not rational to believe it, or to believe it more strongly than previously, or to prefer it to any other unrefuted theory. That problem, never answered, is the one that makes Popper the true godfather of the irrationalist camp.”

  13. Dear Dr. Briggs:

    You are, of course, quite right about the nature of the so-called ” Precautionary Principle .” However, the people trying to invoke this principle as an argument for immediate and draconian action on their agendas admit of no uncertainties with respect to the reality or otherwise of the impending catastrophe du jour – for them P(E) approaches 1, no contingencies, no doubts., and no facts needed. Within their belief system the argument they make therefore makes sense.

  14. Doug M

    It is a little bit like Pascal’s wager.

    If P(God exists) > 0 and the cost for denying that God exists is eternal damnation (infinite). Then the rational decision is to believe.

  15. Ray

    I always found supporters of the precautionary principle to be logically inconsistent. To be logically consistent they should determine that invoking the precautionary principle would not cause any harm before they use it but they don’t.

  16. The take home message is if someone invokes the Precautionary Principle, this means they are unable to do a Cost Benefit Analysis. The reason why they can’t do a Cost Benefit Analysis is because they have no, or inadequate, information to support their claim.

  17. MattS


    “Matt: Chicken Little had the same problem global warming people have–the sky is falling but the rate is so slow it’s not a threat. ”

    No, as with global warming, the real threat is what happens when the (sky stops falling/Climate starts to cool down).

  18. max

    Well the precautionary principal I learned is that if the cost of precaution is minimal, then you do not need to know the actual probability of an event apply the precautionary principal. If, for example, the cost to prevent Catastrophic Anthropogenic Global Climate Change for the next 3 centuries was something minimal, like Lorrie David must only drive electric cars, then we should take the precaution no matter how infinitesimal the probability of Catastrophic Anthropogenic Global Climate Change really is because the 6+ billion people in the word can easily afford to chip in and buy Lorrie David a new electric car every year the rest of her life (heck, I’m in a generous mood, I’ll cover a million people who don’t want to chip in, that’ll be what, a nickle a day?). If the price of prevention is more than minimal, say limiting the world to a 5% increase in CO2 emissions per decade for the next 300 years to prevent a collision with an extrasolar object, then the precautionary principal does not apply and we need to know how likely an event is as well as the effects in order to determine if precaution is a good idea.

    The precautionary principal I learned was based on a different balance entirely, when the cost of just figuring out how likely an event is and how harmful it will be is orders of magnitude less than the cost of prevention, take the preventative measure. The classic example is strep throat, if a course of penicillin costs $0.89 while a throat culture to see if it is strep costs $89.99, the precautionary principal says prescribe the penicillin for anything that looks like strep throat. The precautionary principal I learned does not say start someone on a quarter-of-a-million-dollar experimental drug cocktail with a 25% mortality rate on the miniscule chance that strep throat might be a sign of lupus.

    Argh, just venting about how the precautionary principal has warped in meaning. I like the old precautionary principal, it may have killed a few people and harmed several more (penicillin allergies, non-diagnosed serious diseases and such) but it is fairly simple to use and understand.

  19. JH

    Just cross your fingers or just believe that the future can be achieved on the basis of prayers; everything surely will be fine and dandy.

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