You're Entitled to Arguments, But Not (That Particular) Proof

by Eliezer Yudkowsky8 min read15th Feb 2010228 comments


Motivated ReasoningEvolutionReligionClimate Change

Followup toLogical Rudeness

"Modern man is so committed to empirical knowledge, that he sets the standard for evidence higher than either side in his disputes can attain, thus suffering his disputes to be settled by philosophical arguments as to which party must be crushed under the burden of proof."
        -- Alan Crowe

There's a story - in accordance with Poe's Law, I have no idea whether it's a joke or it actually happened - about a creationist who was trying to claim a "gap" in the fossil record, two species without an intermediate fossil having been discovered.  When an intermediate species was discovered, the creationist responded, "Aha!  Now there are two gaps."

Since I'm not a professional evolutionary biologist, I couldn't begin to rattle off all the ways that we know evolution is true; true facts tend to leave traces of themselves behind, and evolution is the hugest fact in all of biology.  My specialty is the cognitive sciences, so I can tell you of my own knowledge that the human brain looks just like we'd expect it to look if it had evolved, and not at all like you'd think it would look if it'd been intelligently designed.  And I'm not really going to say much more on that subject.  As I once said to someone who questioned whether humans were really related to apes:  "That question might have made sense when Darwin first came up with the hypothesis, but this is the twenty-first century.  We can read the genes.  Human beings and chimpanzees have 95% shared genetic material.  It's over."

Well, it's over, unless you're crazy like a human (ironically, more evidence that the human brain was fashioned by a sloppy and alien god).  If you're crazy like a human, you will engage in motivated cognition; and instead of focusing on the unthinkably huge heaps of evidence in favor of evolution, the innumerable signs by which the fact of evolution has left its heavy footprints on all of reality, the uncounted observations that discriminate between the world we'd expect to see if intelligent design ruled and the world we'd expect to see if evolution were true...

...instead you search your mind, and you pick out one form of proof that you think evolutionary biologists can't provide; and you demand, you insist upon that one form of proof; and when it is not provided, you take that as a refutation.

You say, "Have you ever seen an ape species evolving into a human species?"  You insist on videotapes - on that particular proof.

And that particular proof is one we couldn't possibly be expected to have on hand; it's a form of evidence we couldn't possibly be expected to be able to provide, even given that evolution is true.

Yet it follows illogically that if a video tape would provide definite proof, then, likewise, the absence of a videotape must constitute definite disproof.  Or perhaps just render all other arguments void and turn the issue into a mere matter of personal opinion, with no one's opinion being better than anyone else's.

So far as I can tell, the position of human-caused global warming (anthropogenic global warming aka AGW) has the ball.  I get the impression there's a lot of evidence piled up, a lot of people trying and failing to poke holes, and so I have no reason to play contrarian here.  It's now heavily politicized science, which means that I take the assertions with a grain of skepticism and worry - well, to be honest I don't spend a whole lot of time worrying about it, because (a) there are worse global catastrophic risks and (b) lots of other people are worrying about AGW already, so there are much better places to invest the next marginal minute of worry.

But if I pretend for a moment to live in the mainstream mental universe in which there is nothing scarier to worry about than global warming, and a 6 °C (11 °F) rise in global temperatures by 2100 seems like a top issue for the care and feeding of humanity's future...

Then I must shake a disapproving finger at anyone who claims the state of evidence on AGW is indefinite.

Sure, if we waited until 2100 to see how much global temperatures increased and how high the seas rose, we would have definite proof.  We would have definite proof in 2100, however, and that sounds just a little bit way the hell too late.  If there are cost-effective things we can do to mitigate global warming - and by this I don't mean ethanol-from-corn or cap-and-trade, more along the lines of standardizing on a liquid fluoride thorium reactor design and building 10,000 of them - if there's something we can do about AGW, we need to do it now, not in a hundred years.

When the hypothesis at hand makes time valuable - when the proposition at hand, conditional on its being true, means there are certain things we should be doing NOW - then you've got to do your best to figure things out with the evidence that we have.  Sure, if we had annual data on global temperatures and CO2 going back to 100 million years ago, we would know more than we do right now.  But we don't have that time-series data - not because global-warming advocates destroyed it, or because they were neglectful in gathering it, but because they couldn't possibly be expected to provide it in the first place.  And so we've got to look among the observations we can perform, to find those that discriminate between "the way the world could be expected to look if AGW is true / a big problem", and "the way the world would be expected to look if AGW is false / a small problem".  If, for example, we discover large deposits of frozen methane clathrates that are released with rising temperatures, this at least seems like "the sort of observation" we might be making if we live in the sort of world where AGW is a big problem.  It's not a necessary connection, it's not sufficient on its own, it's something we could potentially also observe in a world where AGW is not a big problem - but unlike the perfect data we can never obtain, it's something we can actually find out, and in fact have found out.

Yes, we've never actually experimented to observe the results over 50 years of artificially adding a large amount of carbon dioxide to the atmosphere.  But we know from physics that it's a greenhouse gas.  It's not a privileged hypothesis we're pulling out of nowhere.  It's not like saying "You can't prove there's no invisible pink unicorn in my garage!"  AGW is, ceteris paribus, what we should expect to happen if the other things we believe are true.  We don't have any experimental results on what will happen 50 years from now, and so you can't grant the proposition the special, super-strong status of something that has been scientifically confirmed by a replicable experiment.  But as I point out in "Scientific Evidence, Legal Evidence, Rational Evidence", if science couldn't say anything about that which has not already been observed, we couldn't ever make scientific predictions by which the theories could be confirmed.  Extrapolating from the science we do know, global warming should be occurring; you would need specific experimental evidence to contradict that.

We are, I think, dealing with that old problem of motivated cognition.  As Gilovich says:  "Conclusions a person does not want to believe are held to a higher standard than conclusions a person wants to believe.  In the former case, the person asks if the evidence compels one to accept the conclusion, whereas in the latter case, the person asks instead if the evidence allows one to accept the conclusion."  People map the domain of belief onto the social domain of authority, with a qualitative difference between absolute and nonabsolute demands:  If a teacher tells you certain things, and you have to believe them, and you have to recite them back on the test.  But when a student makes a suggestion in class, you don't have to go along with it - you're free to agree or disagree (it seems) and no one will punish you.

And so the implicit emotional theory is that if something is not proven - better yet, proven using a particular piece of evidence that isn't available and that you're pretty sure is never going to become available - then you are allowed to disbelieve; it's like something a student says, not like something a teacher says.

You demand particular proof P; and if proof P is not available, then you're allowed to disbelieve.

And this is flatly wrong as probability theory.

If the hypothesis at hand is H, and we have access to pieces of evidence E1, E2, and E3, but we do not have access to proof X one way or the other, then the rational probability estimate is the result of the Bayesian update P(H|E1,E2,E3).  You do not get to say, "Well, we don't know whether X or ~X, so I'm going to throw E1, E2, and E3 out the window until you tell me about X."  I cannot begin to describe how much that is not the way the laws of probability theory work.  You do not get to screen off E1, E2, and E3 based on your ignorance of X!

Nor do you get to ignore the arguments that influence the prior probability of H - the standard science by which, ceteris paribus and without anything unknown at work, carbon dioxide is a greenhouse gas and ought to make the Earth hotter.

Nor can you hold up the nonobservation of your particular proof X as a triumphant refutation.  If we had time cameras and could look into the past, then indeed, the fact that no one had ever "seen with their own eyes" primates evolving into humans would refute the hypothesis.  But, given that time cameras don't exist, then assuming evolution to be true we don't expect anyone to have witnessed humans evolving from apes with our own eyes, for the laws of natural selection require that this have happened far in the distant past.  And so, once you have updated on the fact that time cameras don't exist - computed P(Evolution|~Camera) - and the fact that time cameras don't exist hardly seems to refute the theory of evolution - then you obtain no further evidence by observing ~Video, i.e., P(Evolution|~Video,~Camera) = P(Evolution|~Camera).  In slogan-form, "The absence of unobtainable proof is not even weak evidence of absence."  See appendix for details.

(And while we're on the subject, yes, the laws of probability theory are laws, rather than suggestions.  It is like something the teacher tells you, okay?  If you're going to ignore the Bayesian update you logically have to perform when you see a new piece of evidence, you might as well ignore outright mathematical proofs.  I see no reason why it's any less epistemically sinful to ignore probabilities than to ignore certainties.)

Throwing E1, E2 and E3 out the window, and ignoring the prior probability of H, because you haven't seen unobtainable proof x; or holding up the nonobservation of X as a triumphant refutation, when you couldn't reasonably expect to see X even given that the underlying theory is true; all this is more than just a formal probability-theoretic mistake.  It is logically rude.

After all - in the absence of your unobtainable particular proof, there may be plenty of other arguments by which you can hope to figure out whether you live in a world where the hypothesis of interest is true, or alternatively false.  It takes work to provide you with those arguments.  It takes work to provide you with extrapolations of existing knowledge to prior probabilities, and items of evidence with which to update those prior probabilities, to form a prediction about the unseen.  Someone who does the work to provide those arguments is doing the best they can by you; throwing the arguments out the window is not just irrational, but logically rude.

And I emphasize this, because it seems to me that the underlying metaphor of demanding particular proof is to say as if, "You are supposed to provide me with a video of apes evolving into humans, I am entitled to see it with my own eyes, and it is your responsibility to make that happen; and if you do not provide me with that particular proof, you are deficient in your duties of argument, and I have no obligation to believe you."  And this is, in the first place, bad math as probability theory.  And it is, in the second place, an attitude of trying to be defensible rather than accurate, the attitude of someone who wants to be allowed to retain the beliefs they have, and not the attitude of someone who is honestly curious and trying to figure out which possible world they live in, by whatever signs are available.  But if these considerations do not move you, then even in terms of the original and flawed metaphor, you are in the wrong: you are entitled to arguments, but not that particular proof.

Ignoring someone's hard work to provide you with the arguments you need - the extrapolations from existing knowledge to make predictions about events not yet observed, the items of evidence that are suggestive even if not definite and that fit some possible worlds better than others - and instead demanding proof they can't possibly give you, proof they couldn't be expected to provide even if they were right - that is logically rude.  It is invalid as probability theory, foolish on the face of it, and logically rude.

And of course if you go so far as to act smug about the absence of an unobtainable proof, or chide the other for their credulity, then you have crossed the line into outright ordinary rudeness as well.

It is likewise a madness of decision theory to hold off pending positive proof until it's too late to do anything; the whole point of decision theory is to choose under conditions of uncertainty, and that is not how the expected value of information is likely to work out.  Or in terms of plain common sense:  There are signs and portents, smoke alarms and hot doorknobs, by which you can hope to determine whether your house is on fire before your face melts off your skull; and to delay leaving the house until after your face melts off, because only this is the positive and particular proof that you demand, is decision-theoretical insanity.  It doesn't matter if you cloak your demand for that unobtainable proof under the heading of scientific procedure, saying, "These are the proofs you could not obtain even if you were right, which I know you will not be able to obtain until the time for action has long passed, which surely any scientist would demand before confirming your proposition as a scientific truth."  It's still nuts.


Since this post has already gotten long, I've moved some details of probability theory, the subtext on cryonics, the sub-subtext on molecular nanotechnology, and the sub-sub-subtext on Artificial Intelligence, into:

Demands for Particular Proof:  Appendices.