In the 1840s, at the Vienna General Hospital, there were two maternity wards standing a short walk apart. One was staffed by midwives. The other was staffed by physicians and medical students. In the worst years, women who delivered in the doctor-run ward faced a mortality rate as high as eighteen percent, compared to roughly two percent in the midwife-run ward. Women admitted to the doctor-run ward, once they realized where they had landed, begged on their knees to be moved to the midwife-run ward instead.

A young physician named Ignaz Semmelweis set out to find the cause.

His first guess was posture. Women in the midwife ward tended to give birth on their side, women in the doctor ward on their back. He asked the doctors to change position. The death rate did not move.

His second guess was fear. A priest walked through the doctor-run ward ringing a bell whenever a woman died, and Semmelweis wondered whether the sound itself was killing patients through sheer dread. He asked the priest to take a different route and stop ringing the bell. Again, there was no improvement.

Both guesses were visible, testable, near enough to try. It took a death outside the ward entirely to break the pattern. A colleague of his, a professor named Jakob Kolletschka, died after an accidental cut on his finger during an autopsy. Kolletschka’s own autopsy showed the very signs Semmelweis had seen in the women who died of childbed fever. Semmelweis realized the doctors in his ward were coming directly from performing autopsies to delivering babies, without washing anything off their hands in between. He did not have a word for what was being carried over but he had a mechanism. He ordered every doctor to disinfect their hands with chlorinated lime before examining a patient, every time, no exceptions. Within a year, the mortality rate in the doctor-run ward fell to 1.27 percent, lower than the midwife-run ward’s own rate of 1.33 percent that year.

A story from a Vienna maternity ward in the 1840s matters to someone who has never worked in a hospital and never will, because most people are carrying a version of posture or bell right now, just in something closer to home. An explanation gets picked for why a relationship keeps hitting the same wall, why a habit will not stick, why a project at work keeps stalling right where it always does, and it is rarely picked because it was tested and held up. It is picked because it was the first thing within easy reach, and it survives mostly because nobody has acted on it directly enough for it to fail out loud. Try harder at the habit and it still slips by the second week. Fix the one thing in the relationship that seemed obvious and that fight comes back a month later anyway. Nothing about the explanation was disproven before that. It just had not been tested yet. Posture and the bell sat in that blind spot until someone investigated them and the death rate refused to move. What follows is a way of catching that kind of guess earlier.

A separate practice of mine is keeping twelve questions open on purpose rather than settled, ones that stay interesting long after any answer arrives, described in Twelve Questions, Invited in for Tea. Mental models turn out to do two different jobs for a question like that, tools for searching out a mechanism, and a test for whether a question can accept one at all, and Semmelweis’s ward is where both jobs are easiest to see.

What a Mental Model Is

A mental model is an internal representation of how something behaves, built by watching a mechanism operate closely enough to reason with it and predict what happens next, even in situations never directly observed. It does not tell anyone what to do. Knowing that something on unwashed hands could travel from a corpse to a patient is a mental model. It does not say when to wash, or how often, or in what solution. It only says how the transfer happens, in any ward, on any morning.

1. Mental Models as Reasoning Tools

The version most people use day to day is Shane Parrish’s cataloguing of recurring mental models. He starts from the blind spots built into ordinary thinking, then offers general reasoning moves to work around them, such as inversion, first principles, and Occam’s razor, ways of thinking through any problem, not pictures of how one specific thing works. None of the three originate with Parrish, but his cataloguing is where most people meet them, and they are the tools that got Semmelweis to a candidate mechanism in the first place.

Inversion. Inversion turns a question around: instead of asking what is causing a problem, ask what is different about the one situation where the problem does not happen. In Semmelweis’s ward, the natural question was what is killing the doctors’ patients. The more useful question, and the one Semmelweis was slow to ask, was why do the midwives’ patients keep living. Posture and the bell were both still answers to the first question. Turning the question around did not hand him the mechanism directly, but it pointed him at the right comparison, the one ward that was somehow not a problem, rather than only interrogating the one that was.

First principles. First principles means working from what can be directly observed rather than from whatever explanation is assumed to be true. The convention of Semmelweis’s time was miasma theory, the idea that bad air in general caused disease. Starting from that convention would have kept him looking at ventilation, crowding, the general atmosphere of the ward. Starting instead from what was actually observed, one ward killing at a much higher rate than the other with nothing else obviously different between them, is what put the miasma explanation aside entirely rather than trying to patch it.

Occam’s razor. Occam’s razor takes the simplest account that fits everything observed, not the first explanation that happens to be within reach. Posture and the bell are often misremembered as the simple explanations, discarded in favor of something more complicated. They were not simple, they were just convenient. The simplest explanation consistent with everything observed was the one difference nobody had thought to look at, what the doctors were doing with their hands in the hour before they walked in.

These three moves generalize past this one hospital. When an explanation for a stalled habit or a recurring argument feels obvious the moment it is reached for, invert the question first: instead of asking what is causing the problem, ask what is different about the one version of the situation where it does not happen. Reason from what can be observed rather than from whatever explanation you have simply taken for granted. And when two explanations both fit, take the simpler one, not the one that happens to be the most readily available.

2. Mental Models as a Sorting Test

The second is different: a mental model can also test whether a question is the kind that has a mechanism to find in the first place, which is where the twelve favorite questions come back in.

The mechanism, once verified, opened onto a different set of questions entirely. Putting myself in his shoes, here is what five of my own twelve favorite questions, using the numbering from the original list, would have asked next.

Question 7. How to shape an idea so it is ready to be received? He had the data. He published it, sent copies to prominent physicians across Europe, and watched most of them dismiss it anyway. Having the answer, and in retrospect the right answer, did not mean he had found a way to make anyone else hold it.

Question 5. Given any problem, how to find the question worth answering? The diagnostic question, what is killing these women, he answered in months. The question he never seemed to resolve was different: whether the fight worth having was convincing Vienna’s medical establishment, or simply running his own ward correctly and letting the results speak for themselves. Between the two, he kept choosing to convince the establishment, rather than letting his ward’s own numbers make the case.

Question 4. What is the objective function? He said, more than once, that he wanted no personal credit, only the practical outcome of a mother keeping her child. But the letters he wrote later are furious and accusatory in a way that reads like a man chasing something else too, being heard, being vindicated, maybe just being believed by someone. Wanting only the outcome and needing to be personally vindicated pull in different directions, and it is not obvious which one he was actually optimizing for.

Question 12. Who are his people? Johann Klein, his own superior, looked at a falling mortality rate and reached for the same miasma logic Semmelweis had already set aside: credit went to a new ventilation system. The ward’s ventilation had not changed at all. Klein called it the cause anyway, without asking what had actually changed on the ward: the doctors were washing their hands. Klein held real power over Semmelweis’s career and used it. When Semmelweis’s assistant professorship came up for renewal in 1849, Klein did not renew it. Whoever Klein was to him, he was not one of Semmelweis’s people, and losing him meant losing the answer to who counted as his people at all. By then, the men who trained him had stopped weighing what he had found. They were managing him instead, treating the man himself as the difficulty to be dealt with.

Question 11. What does he owe the world? Physicians across Europe kept dismissing the idea, year after year, even with the book in hand. He knew something that could save lives, and the people with the power to act on it would not. None of it resolved. His mind gave out under the weight of carrying it, and he died in an asylum in 1865. The idea itself was accepted afterward, once germ theory caught up to what he had already found. He was not alive to know it.

Not all five of these resist equally, and separating which do from which do not matters here. Some have an actual mechanism to grip, something with interlocking parts, not sitting side by side, something that still explains a case it was never built for. Others do not, no matter how carefully the description was built.

Questions 4 and 11, what is the objective function and what does he owe the world, are values questions: they have nothing for a mechanism to grip in the first place. There is no candidate explanation to test, because there is nothing describing how something works, only a claim about what ought to be wanted or what is owed. A perfectly built model of how credit and recognition operate inside a medical hierarchy would still not tell Semmelweis whether he should have wanted vindication or only outcomes. That is a decision, not a description, and no mechanism, however well tested, was ever going to settle it.

Questions 7 and 12 are a different case: call them mechanism questions. This is where a real explanation holds up. Take who are his people. A candidate mechanism is available: people defend a position when the evidence backing it outweighs the cost to their own standing, and abandon it when the cost outweighs the evidence. That is two things in tension, evidence on one side, reputational cost on the other, and it does not only describe Klein, it describes any colleague deciding whether to defend or abandon a position once their own name is attached to being wrong. It does not decide who Semmelweis’s people were. It describes how people in his position tend to behave once their own standing is on the line, which is close enough to an answer that a person could act on directly. Shape an idea so it is ready to be received holds up just as well: an idea’s chance of adoption comes down to how much it threatens existing authority weighed against how credible its messenger already is, and that holds well beyond a nineteenth-century ward. Question 5, given any problem how to find the one worth answering, sits in between, part describable pattern, part a matter of worth that no description ever settles.

A guess and a mechanism can feel identical from the inside. The difference only surfaces once one of them gets tested and the other does not. Posture and fear felt exactly as reasonable to Semmelweis, in the moment, as cadaverous particles did once he found them. Some of my own twelve behave like 7 and 12, mechanism questions informed by something substantial nearly to the point of answered. Others behave like 4 and 11, values questions that no mechanism will ever close, because they were never asking how something works to begin with. What a real explanation does instead, for a question in that second group, is not close it. It gives the question solid ground to stand on instead of instinct and hurt feelings alone. The question about Klein got exactly that, finally getting asked in specific, checkable terms rather than staying a vague grievance. A guess wearing a mental model’s clothes cannot do that. It only feels like it can. That confidence lasts exactly until someone tests it. What matters is not whether a favorite question ever gets answered. It is whether the ones that stay open get engaged with something concrete.

None of this needs a hospital, or a hundred and eighty years of hindsight, to put to use. It needs being honest about which explanation currently running a person’s own decisions is a posture guess and which is an actual mechanism, and being willing to find out which is which before the cost of being wrong compounds into what it cost him.

A Closing Reflection. Posture and the bell felt just as reasonable as the real cause. Testing them is what took that feeling apart. The four guesses below are a chance to run that test on something smaller and much more ordinary.

  1. Where do you guess instead of measure, and get away with it most of the time: timing how long something will take, how much to pack, how much to say in a message before it reads as too much or too little, how long to wait before following up on something.
  2. Where has someone else corrected your approximation, and the correction is what stayed rather than the original guess: a moment someone said no, it is not like that, here is why, and the reason stuck with you afterward.
  3. Where do you approximate on purpose, knowing you are doing it, because the real thing does not earn precision, versus where you approximate without noticing and only find out later that it cost you something.
  4. Where do you approximate specifically because you do not care to do otherwise, not from ignorance but from a deliberate choice not to invest attention there, a slightly more honest version of eyeballing than the usual story of someone who wanted precision and did not know how to get it.

Sit with those for a minute rather than answering right away. The one that gives a specific afternoon, a specific voice, a specific object in hand is the one worth zooming into. Sorting out which one it is isn’t the mental model itself. It is the first step toward one. Noticing that the two wards differed only in what the doctors did with their hands was that kind of first step, not yet the germ theory that followed. Search for the mechanism using the three tools that found one here: inversion, first principles, Occam’s razor. Once recognized as one, it is worth continuing to press on.

Where This Practice Came From

Craik proposed in 1943 that the mind builds small working models of reality to test outcomes before acting, and Johnson-Laird’s later work kept the core of it: a mental model is a representation used to reason and predict, not just a summary, one whose parts constrain each other and that still holds up in a case it was not built for. Parrish’s cataloguing of recurring mental models, drawn together across his Farnam Street work, is where most people encounter tools like inversion, first principles, and Occam’s razor today, though none of the three originate with him. Senge’s The Fifth Discipline named mental models as one of the disciplines of a learning organization, the often unexamined images of how the world works that quietly shape action.

Intellectual Honesty Note. The mechanism question and values question naming, and the framing of mental models as doing two separate jobs, a reasoning tool and a sorting test, is my own construction, not inherited directly from Craik, Johnson-Laird, Parrish, or Senge.

References

Craik, K. J. W. (1943). The Nature of Explanation. Cambridge University Press.

Gombos, Róza. “Ignaz Semmelweis.” Science History Institute, September 15, 2025. https://www.sciencehistory.org/education/scientific-biographies/ignaz-semmelweis/

Johnson-Laird, P. N. (1983). Mental Models: Towards a Cognitive Science of Language, Inference, and Consciousness (No. 6). Harvard University Press.

Johnson-Laird, P. N., Girotto, V., & Legrenzi, P. (1998). Mental Models: a Gentle Guide for Outsiders. Sistemi intelligenti, 9(68), 33.

Johnston, Katrina. “Ignaz Semmelweis: The Saviour of Mothers.” Canadian Museum of Health Care, 2022. https://www.museumofhealthcare.ca/blog/ignaz-semmelweis-the-saviour-of-mothers

Parrish, S. (2019). The Great Mental Models: General Thinking Concepts. Latticework Publishing.

Senge, P. M. (2006). The Fifth Discipline: The Art and Practice of the Learning Organization. Broadway Business.