Falsifiability: why you rule things out, not in

This June, I had the honor of speaking at O’Reilly Velocity 2016 in Santa Clara. My topic was Troubleshooting Without Losing Common Ground, which I’ve written about and written about before that too.

I was pretty happy with my talk, especially the Star Trek: The Next Generation vignette in the middle. It was a lot of ideas to pack into a single talk, but I think a lot of people got the point. However, I did give a really unsatisfactory answer (30m46s) to the first question I received. The question was:

In the differential diagnosis steps, you listed performing tests to falsify assumptions. Are you borrowing that from medicine? In tech are we only trying to falsify assumptions, or are we sometimes trying to validate them?

I didn’t have a real answer at the time, so I spouted some bullshit and moved on. But it’s a good question, and I’ve thought more about it, and I’ve come up with two (related) answers: a common-sense answer and a pretentious philosophical answer.

The Common Sense Answer

My favorite thing about differential diagnosis is that it keeps the problem-solving effort moving. There’s always something to do. If you’re out of hypotheses, you come up with new ones. If you finish a test, you update the symptoms list. It may not always be easy to make progress, but you always have a direction to go, and everybody stays on the same page.

But when you seek to confirm your hypotheses, rather than to falsify others, it’s easy to fall victim to tunnel vision. That’s when you fixate on a single idea about what could be wrong with the system. That single idea is all you can see, as if you’re looking at it through a tunnel whose walls block everything else from view.

Tunnel vision takes that benefit of differential diagnosis – the constant presence of a path forward – and negates it. You keep running tests to try to confirm your hypothesis, but you may never prove it. You may just keep getting tests results that are consistent with what you believe, but that are also consistent with an infinite number of hypotheses you haven’t thought of.

A focus on falsification instead of verification can be seen as a guard against tunnel vision. You can’t get stuck on a single hypothesis if you’re constrained to falsify other ones. The more alternate hypotheses you manage to falsify, the more confident you get that you should be treating for the hypotheses that might still be right.

Now, of course, there are times when it’s possible to verify your hunch. If you have a highly specific test for a problem, then by all means try it. But in general it’s helpful to focus on knocking down hypotheses rather than propping them up.

The Pretentious Philosophical Answer

I just finished Karl Popper’s ridiculously influential book The Logic of Scientific Discovery. If you can stomach a dense philosophical tract, I would highly recommend it.

Karl “Choke Right On It, Logical Positivism” Popper

Published in 1959 – but based on Popper’s earlier book Logik der Forschung from 1934 – The Logic Of Scientific Discovery makes a then-controversial [now widely accepted (but not universally accepted, because philosophers make cats look like sheep, herdability-wise)] claim. I’ll paraphrase the claim like so:

Science does not produce knowledge by generalizing from individual experiences to theories. Rather, science is founded on the establishment of theories that prohibit classes of events, such that the reproducible occurrence of such events may falsify the theory.

Popper was primarily arguing against a school of thought called logical positivism, whose subscribers assert that a statement is meaningful if and only if it is empirically testable. But what matters to our understanding of differential diagnosis isn’t so much Popper’s absolutely brutal takedown of logical positivism (and damn is it brutal), as it is his arguments in favor of falsifiability as the central criterion of science.

I find one particular argument enlightening on the topic of falsification in differential diagnosis. It hinges on the concept of self-contradictory statements.

There’s an important logical precept named – a little hyperbolically – the Principle of Explosion. It asserts that any statement that contradicts itself (for example, “my eyes are brown and my eyes are not brown”) implies all possible statements. In other words: if you assume that a statement and its negation are both true, then you can deduce any other statement you like. Here’s how:

  1. Assume that the following two statements are true:
    1. “All cats are assholes”
    2. “There exists at least one cat that is not an asshole”
  2. Therefore the statement “Either all cats are assholes, or 9/11 was an inside job” (we’ll call this Statement A) is true, since the part about the asshole cats is true.
  3. However, if the statement “there exists at least one cat that is not an asshole” is true too (which we’ve assumed it is) and 9/11 were not an inside job, then Statement A would be false, since neither of its two parts would be true.
  4. So the only way left for Statement A to be true is for “9/11 was an inside job” to be a true statement. Therefore, 9/11 was an inside job.
  5. Wake up, sheeple.


The Principle of Explosion is the crux of one of Popper’s most convincing arguments against the Principle of Induction as the basis for scientific knowledge.

It was assumed by many philosophers of science before Popper that science relied on some undefined Principle of Induction which allowed one to generalize from a finite list of experiences to a general rule about the universe. For example, the Principle of Induction would allow one to deduce from enough statements like “I dropped a ball and it fell” and “My friend dropped a wrench and it fell” to “When things are dropped, they fall.” But Popper argued against the existence of the Principle of Induction. In particular, he pointed out that:

If there were some way to prove a general rule by demonstrating the truth of a finite number of examples of its consequences, then we would be able to deduce anything from such a set of true statements.

Right? By the Principle of Explosion, a self-contradictory statement implies the truth of all statements. If we accepted the Principle of Induction, then the same evidence that proves “When things are dropped, they fall” would also prove “All cats are assholes and there exists at least one cat that is not an asshole,” which would prove every statement we can imagine.

So what does this have to do with falsification in differential diagnosis? Well, imagine you’ve come up with these hypotheses to explain some API slowness you’re troubleshooting:

Hypothesis Alpha: contention on the table cache is too high, so extra latency is introduced for each new table opened

Hypothesis Bravo: we’re hitting our IOPS limit on the EBS volume attached to the database server

There are many test results that would be compatible with Hypothesis Alpha. But unless you craft your tests very carefully, those same results will also be compatible with Hypothesis Bravo. Without a highly specific test for table cache contention, you can’t prove Hypothesis Alpha through a series of observations that agree with it.

What you can do, however, is try to quickly falsify Hypothesis Bravo by checking some graphs against some AWS configuration data. And if you do that, then Hypothesis Alpha is the your best remaining guess. Now you can start treating for table cache contention on the one hand, and attempting the more time-consuming process (especially if it’s correct!) of falsifying Hypothesis Alpha.

Isn’t this kind of abstract?

Haha OMG yes. It’s the most abstract. But that doesn’t mean it’s not a useful idea.

If it’s your job to troubleshoot problems, you know that tunnel vision is very real. If you focus on generating alternate hypotheses and falsifying them, you can resist tunnel vision’s allure.

One thought on “Falsifiability: why you rule things out, not in

  1. Thomas

    Perhaps I’m restating some of the things you said above, but If I could add a few more thoughts. . .
    1. In order to keep ruling out diagnoses, those diagnoses have to be falsifiable/disprovable. If they are not, one can wind up with the same “tunnel vision” where it’s only possible to confirm the diagnosis. In medicine this can lead to serious consequences. (see Emma Eckstein and Sigmund Freud)
    2. It can take forever to try to confirm a diagnosis, but it often takes only minutes to falsify it. So even if you are considering only a single diagnosis, you can save a ton of time by focusing first on ruling it out (falsification).
    3. Not only can confirmation take forever, an infinite number of diagnoses can all appear to be confirmed if no-one ever attempts to disprove them, or if the diagnoses are un-falsifiable.
    You see this happening in religions, which are largely un-falsifiable: every group in the world came up with a different explanation for what they saw around them, and each group is still convinced that their explanation is the one that is correct.

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