Category: Science

Thomas Kuhn: The Structure of Scientific Revolutions

“The decision to reject one paradigm is always simultaneously the decision to accept another, and the judgment leading to that decision involves the comparison of both paradigms with nature and with each other.”

structure of scientific revolutions

The progress of science is commonly perceived of as a continuous, incremental advance, where new discoveries add to the existing body of scientific knowledge. This view of scientific progress, however, is challenged by the physicist and philosopher of science Thomas Kuhn, in his book The Structure of Scientific Revolutions. Kuhn argues that the history of science tells a different story, one where science proceeds with a series of revolutions interrupting normal incremental progress.

“A prevailing theory or paradigm is not overthrown by the accumulation of contrary evidence,” Richard Zeckhauser wrote, “but rather by a new paradigm that, for whatever reasons, begins to be accepted by scientists.”

Between scientific revolutions, old ideas and beliefs persist. These form the barriers of resistance to alternative explanations.

Zeckhauser continues “In this view, scientific scholars are subject to status quo persistence. Far from being objective decoders of the empirical evidence, scientists have decided preferences about the scientific beliefs they hold. From a psychological perspective, this preference for beliefs can be seen as a reaction to the tensions caused by cognitive dissonance. ”

* * *

Gary Taubes posted an excellent blog post discussing how paradigm shifts come about in science. He wrote:

…as Kuhn explained in The Structure of Scientific Revolutions, his seminal thesis on paradigm shifts, the people who invariably do manage to shift scientific paradigms are “either very young or very new to the field whose paradigm they change… for obviously these are the men [or women, of course] who, being little committed by prior practice to the traditional rules of normal science, are particularly likely to see that those rules no longer define a playable game and to conceive another set that can replace them.”

So when a shift does happen, it’s almost invariably the case that an outsider or a newcomer, at least, is going to be the one who pulls it off. This is one thing that makes this endeavor of figuring out who’s right or what’s right such a tricky one. Insiders are highly unlikely to shift a paradigm and history tells us they won’t do it. And if outsiders or newcomers take on the task, they not only suffer from the charge that they lack credentials and so credibility, but their work de facto implies that they know something that the insiders don’t – hence, the idiocy implication.

…This leads to a second major problem with making these assessments – who’s right or what’s right. As Kuhn explained, shifting a paradigm includes not just providing a solution to the outstanding problems in the field, but a rethinking of the questions that are asked, the observations that are considered and how those observations are interpreted, and even the technologies that are used to answer the questions. In fact, often the problems that the new paradigm solves, the questions it answers, are not the problems and the questions that practitioners living in the old paradigm would have recognized as useful.

“Paradigms provide scientists not only with a map but also with some of the direction essential for map-making,” wrote Kuhn. “In learning a paradigm the scientist acquires theory, methods, and standards together, usually in an inextricable mixture. Therefore, when paradigms change, there are usually significant shifts in the criteria determining the legitimacy both of problems and of proposed solutions.”

As a result, Kuhn said, researchers on different sides of conflicting paradigms can barely discuss their differences in any meaningful way: “They will inevitably talk through each other when debating the relative merits of their respective paradigms. In the partially circular arguments that regularly result, each paradigm will be shown to satisfy more or less the criteria that it dictates for itself and to fall short of a few of those dictated by its opponent.”

But Taubes’ explanation wasn’t enough to satisfy my curiosity.


The Structure of Scientific Revolutions

To learn more on how paradigm shifts happen, I purchased Kuhn’s book, The Structure of Scientific Revolutions, and started to investigate.

Kuhn writes:

“The decision to reject one paradigm is always simultaneously the decision to accept another, and the judgment leading to that decision involves the comparison of both paradigms with nature and with each other.”

Anomalies are not all bad.

Yet any scientist who pauses to examine and refute every anomaly will seldom get any work done.

…during the sixty years after Newton’s original computation, the predicted motion of the moon’s perigee remained only half of that observed. As Europe’s best mathematical physicists continued to wrestle unsuccessfully with the well-known discrepancy, there were occasional proposals for a modification of Newton’s inverse square law. But no one took these proposals very seriously, and in practice this patience with a major anomaly proved justified. Clairaut in 1750 was able to show that only the mathematics of the application had been wrong and that Newtonian theory could stand as before. … persistent and recognized anomaly does not always induce crisis. … It follows that if an anomaly is to evoke crisis, it must usually be more than just an anomaly.

So what makes an anomaly worth the effort of investigation?

To that question Kuhn responds, “there is probably no fully general answer.” Einstein knew how to sift the essential from the non-essential better than most.

When the anomaly comes to be recognized as more than another puzzle of science the transition, or revolution, has begun.

The anomaly itself now comes to be more generally recognized as such by the profession. More and more attention is devoted to it by more and more of the field’s most eminent men. If it still continues to resist, as it usually does not, many of them may come to view its resolution as the subject matter of their discipline. …

Early attacks on the anomaly will have followed the paradigm rules closely. As time passes and scrutiny increases, more of the attacks will start to diverge from the existing paradigm. It is “through this proliferation of divergent articulations,” Kuhn argues, “the rules of normal science become increasing blurred.

Though there still is a paradigm, few practitioners prove to be entirely agreed about what it is. Even formally standard solutions of solved problems are called into question.”

Einstein explained this transition, which is the structure of scientific revolutions, best. He said: “It was as if the ground had been pulled out from under one, with no firm foundation to be seen anywhere, upon which one could have built.

All scientific crises begin with the blurring of a paradigm.

In this respect research during crisis very much resembles research during the pre-paradigm period, except that in the former the locus of difference is both smaller and more clearly defined. And all crises close in one of three ways. Sometimes normal science ultimately proves able to handle the crisis—provoking problem despite the despair of those who have seen it as the end of an existing paradigm. On other occasions the problem resists even apparently radical new approaches. Then scientists may conclude that no solution will be forthcoming in the present state of their field. The problem is labelled and set aside for a future generation with more developed tools. Or, finally, the case that will most concern us here, a crisis may end up with the emergence of a new candidate for paradigm and with the ensuing battle over its acceptance.

But this isn’t easy.

The transition from a paradigm in crisis to a new one from which a new tradition of normal science can emerge is far from a cumulative process, one achieved by an articulation or extension of the old paradigm. Rather it is a reconstruction of the field from new fundamentals, a reconstruction that changes some of the field’s most elementary theoretical generalizations as well as many of its paradigm methods and applications.

Who solves these problems? Do the men and women who have invested a large portion of their lives in a field or theory suddenly confront evidence and change their mind? Sadly, no.

Almost always the men who achieve these fundamental inventions of a new paradigm have been either very young, or very new to the field whose paradigm they change. And perhaps that point need not have been made explicit, for obviously these are men who, being little committed by prior practice to the traditional rules of normal science, are particularly likely to see that those rules no longer define a playable game and to conceive another set that can replace them.


Therefore, when paradigms change, there are usually significant shifts in the criteria determining the legitimacy both of problems and of proposed solutions.

That observation returns us to the point from which this section began, for it provides our first explicit indication of why the choice between competing paradigms regularly raises questions that cannot be resolved by the criteria of normal science. To the extent, as significant as it is incomplete, that two scientific schools disagree about what is a problem and what is a solution, they will inevitably talk through each other when debating the relative merits of their respective paradigms. In the partially circular arguments that regularly result, each paradigm will be shown to satisfy more or less the criteria that it dictates for itself and to fall short of a few of those dictated by its opponent. There are other reasons, too, for the incompleteness of logical contact that consistently characterizes paradigm debates. For example, since no paradigm ever solves all the problems it defines and since no two paradigms leave all the same problems unsolved, paradigm debates always involve the question: Which problems is it more significant to have solved? Like the issue of competing standards, that questions of values can be answered only in terms of criteria that lie outside of normal science altogether.

Many years ago Max Planck offered this insight: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”

If you’re interested in learning more about how paradigm shifts happen, read The Structure of Scientific Revolutions.

David Quammen on Why Big Populations Survive and Small Ones Go Extinct

“Big populations don’t go extinct. Small populations do.
It’s not a surprising finding but it is a significant one.”


Why do small populations go extinct?

While the answer is simple to outline the scientific details are more nuanced. For now, lets stick to the outline version.

“Small populations go extinct because (1) all populations fluctuate in size from time to time, under the influence of two kinds of factors, which ecologists refer to as deterministic and stochastic; and (2) small populations, unlike big ones, stand a good chance of fluctuation to zero, since zero is not far away.”

song of the dodo

Deterministic factors are those involving straightforward cause-and-effect relations that to some extent can be predicted and controlled: hunting, trapping, destroying habitat, introducing new animals that compete with or prey on existing ones, etc.

Stochastic factors “operate in a realm beyond human prediction and control, either because they are truly random or because they are linked to geophysical or biological causes so obscurely complex that they seem random.” We’re talking things like weather patterns, epidemic disease, infestation of parasites, forest fires, etc. Each might cause a downward fluctuation in the population of some species.

In Song of the Dodo, David Quammen gives the following illuminating example.

Think of two species that live on the same tiny island. One is a mouse. Total population, ten thousand. The other is an owl. Total population, eighty. The owl is a fierce and proficient mouse eater. The mouse is timorous, fragile, easily victimized. But the mouse population as a collective entity enjoys the security of numbers.

Say that a three-year drought hits the island of owls and mice, followed by a lightning-set fire, accidental events that are hurtful to both species. The mouse population drops to five thousand, the owl population to forty. At the height of the next breeding season a typhoon strikes, raking the treetops and killing and entire generation of unfledged owls. Then a year passes peacefully, during which the owl and the mouse populations both remain steady, with attrition from old age and individual mishaps roughly offset by new births. Next, the mouse suffers an epidemic disease, cutting its population to a thousand, fewer than at any other time within decades. This extreme slump even affects the owl, which begins starving for lack of prey.

Weakened by hunger, the owl suffers its own epidemic, from a murderous virus. Only fourteen birds survive. Just six of those fourteen owls are female, and three of the six are too old to breed. Then a young female owl chokes to death on a mouse. That leaves two fertile females. One of them loses her next clutch of eggs to a snake. The other nests successfully and manages to fledge four young, all four of which happen to be male. The owl population is now depressed to a point of acute vulnerability. Two breeding females, a few older females, a dozen males. Collectively they possess insufficient genetic diversity for adjusting to further troubles, and there is a high chance of inbreeding between mothers and sons. The inbreeding, when it occurs, tends to yield some genetic defects. Meanwhile the mouse population is also depressed far below its original number.

Ten years pass, with the owl population becoming progressively less healthy because of inbreeding. A few further females are hatched, precious additions to the gender balance, though some of them turn out to be congenitally infertile. During that same stretch of time the mouse population rebounds vigorously. Good weather, plenty of food, no epidemics, genetically it’s fine—and so the mouse quickly returns to its former abundance.

Then another wildfire scorches the island, killing four adult owls, and, oh, six thousand mice. The four dead owls were all breeding-age females, crucial to the beleaguered population. The six thousand mice were demographically less crucial. Among the owls there now remains only one female who is young and fertile. She develops ovarian cancer, a problem to which she is susceptible because of the history of inbreeding among her ancestors. She dies without issue. Very bad news for the owl species. Let’s give the mouse another plague of woe, just to be fair: a respiratory infection, contagious and lethal, causes eight hundred fatalities. None of this is implausible. These things happen. The owl population—reduced to a dozen mopey males, several dowagers, no fertile females—is doomed to extinction. When the males and the dowagers die off, one by one, leaving not offspring, that’s that. The mouse population fluctuates upward in response to the extinction of the owls, a rude signal that life is easier in the absence of predation. Twelve thousand mice. Fifteen thousand. Twenty thousand. But while its numbers are so high it will probably overexploit its own resources and eventually decline again as a consequence of famine. Then rise again. Then decline again. Then …

The mouse population is a yo-yo on a long string. Despite all the accidental disasters, despite all the ups and downs, the mouse doesn’t go extinct because the mouse is not rare. The owl goes extinct. Why? Because life is a gauntlet of uncertainties and the owl’s population size, in the best of times, was too small to buffer it against the worst of times.

Still curious? Read The Song of the Dodo.

Why is it so Hard to Kill a Cockroach with your Shoe?

The Cockroach Papers by Richard Schwied is an interesting book if you are looking to learn more about biology or evolution. Cockroaches are built for survival no matter what the world throws at them. Their ability to adapt is just amazing.

Here are some of my notes from the book.

Food and Water
German cockroaches, Blattella germanica, the most common domestic roach in the United States, have been observed to live 45 days without food, and more than two weeks with neither food nor water.

Cockroaches will eat almost anything including glue, feces, hair, decayed leaves, paper, leather, banana skins, other cockroaches, and dead or alive humans. They will not, however, eat cucumbers. They are particularly fond of dried milk around a baby’s mouth.

The roaches are not confined to any particular environment and live in a tremendous variety of places, from underneath woodpiles in Alaska to high in the jungle canopy in the tropics of Costa Rica. They are even found in the caves of Borneo and under the thorn bushes in arid stretches of Kenya. Wherever they live, they are masters at surviving. They are, Schwied writes, “undeniably one of the pinnacles of evolution on this planet.”

Why is it so hard to kill a cockroach with your shoe?
Schweid observes that “when a cockroach feels a breeze stirring the hairs on its cerci, it does not wait around to see what is going to happen next, but leaves off whatever it is doing and goes immediately into escape mode in something remarkably close to instantaneous fashion.” Studies show that a cockroach can respond in about 1/20th of a second, so “by the time a light comes on and human sight can register it, much less react by reaching for and hoisting something with which to squash it, a roach is already locomoting towards safety.”

Cockroach blood is a pigments, clear substance circulating through the interior of its body, and what usually spurts out of a roach when its hard, , outer shell—its exoskeleton—is penetrated or squashed is a cream-colored substance resembling nothing so much as pus or smegma.

Cockroaches have two brains—one inside their skulls, and a second, more primitive brain that is back near their abdomen.

Schweid says “Pheromones, chemical signals of sexual readiness, operate between a male and female cockroach to initiate courtship and copulation. A sexually receptive female assumes a posture with her abdomen lowered and her wings rais and gives off a pheromone that attracts males.” If he finds a virgin female, a male cockroach after some antenna rubbing foreplay will turn away from the female and raise his wings, “an invitation to her to mount.”Copulation frequently lasts an hour. After sex, female cockroaches store the sperm and use them as needed. The sperm may last her a lifetime.

“The evolutionary strategy employed by cockroaches to reproduce is considerably more efficient than that employed by humans.” Oddly, there are certain species of cockroaches that can, at least for a generation or two, reproduce without any sperm. Schweid says “the females unfertilized eggs will develop and hatch—always producing new females.”

Betty Faber, the former staff entomologist for the New York Natural History Museum, says “Females go to bed—by which I mean disappear back to the harborage—at night earlier than males.”

Schweid writes, “cockroaches, while not social insects in the entomological sense of bees or ants with clearly assigned tasks that benefit the whole community, do clearly take pleasure in the company of other roaches, and the aggression pheromones draw them together, eliciting their effects regardless of the sex or age.” Cockroaches reared singly develop more slowly and take longer between molts than do those reared in a group. Although those groups can be too big “just as development is delayed in young cockroaches if they are isolated, over-crowding also extends the time between molts. So there is yet another kind of pheromone, called a “dispersal pheromone,” and it serves as the chemical signal that it is time to look for a new, slightly roomier harborage. This chemical is found in the insects’ saliva, and has just the opposite effect of the aggression attractant, in that it repulses cockroaches and causes them to look elsewhere for harborage.”

In case you’re thinking we can just nuke the little critters you should know that cockroaches survived the atomic bombs test blast at Bikini. “There is such a thing as a lethal dose of radiation for a cockroach, but it is a lot higher than our own.”

“While few humans may eat them, the roach has both external and internal predators and parasites. There are centipedes that have a primary diet of cockroaches. Mantises, ants, and scorpions will eat them, as will a variety of larger animals including toads, frogs, possums, hedgehogs, armadillos, mongooses, monkeys, lizards, spiders, mice, cats, and birds”

Roaches are nocturnal and pass their days sleeping.

Male aggression
“Cockroaches, like so many other species including our own, have male aggression rituals. They have their own inventory of aggressive behaviors, a scale of conflict that begins with threatening postures. Beyond that they graduate to antenna lashing—a form of which is also present in male/female encounters to determine if a female is sexually receptive–and biting. Sex and territory seem to be the primary motivations for fighting between male cockroaches: These clashes never end in death, but always in the retreat of one fighter.”

Trapping a cockroach
“Stale while bread moistened with warm, slightly soured beer” is the most reliable and effective. “This is typically placed at the bottom of a small jar—a Gerber’s baby food jar, say—around the interior rim of which a petroleum jelly like Vaseline has been applied. The cockroach can climb in from the outside but can’t climb back out.”

What should you do if you get a cockroach stuck in your ear?
“It is, according to all accounts, painful and horrifying, although a little mineral oil or lidocaine sprayed into the ear is usually enough to dislodge the intruder.”

Exterminators primarily employ two methods to kill the cockroach: gas and gel. The gel is way more effective but many still rely on the spray. Why? “The major problem that exterminators have with the gel is that it has no immediate knockdown effect.”

John Wickham, an English pest control consultant defined knockdown as: “The inability of the insect to move in a sufficiently coordinated manner to right itself and progress normally.” When a roach eats gel bait—the safer of the two methods—it heads home before the active poison kills it.

“Customers who are paying $75 an hour like to see these roaches struggling to get up, in agony and convulsions, and the sprays, with substantial knockdown effect, provide them that gratifying visual reassurance that the problem is being solved and that they are getting their money’s worth.

It’s unlikely this poison will have much long term impact. “Almost as soon as an effective poison goes into widespread use, cockroaches begin to develop Resistance. And, typically, the most efficacious products developed, those that do the best job, turn out to be more detrimental to our own health than are the roaches.”

If you want to learn more about cockroaches read The Cockroach Papers.

Massively Distilled Wisdom

Richard Feynman famously asked: “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words?”

Fascinated by Feynman’s question, Seed put a similar one to a number of leading thinkers: “Imagine—much as Feynman asked his audience—that in a mission to change everyone’s thinking about the world, you can take only one lesson from your field as a guide. In a single statement, what would it be?” Here are their answers:

“The scale of the human socio-economic-political complex system is so large that it seriously interferes with the biospheric complex system upon which it is wholly dependant, and cultural evolution has been too slow to deal effectively with the resulting crisis.” —Paul R. Ehrlich is president of the Center for Conservation Biology at Stanford University.

“Humans have a tendency to fall prey to the illusion that their economy is at the very center of the universe, forgetting that the biosphere is what ultimately sustains all systems, both man-made and natural. In this sense, ‘environmental issues’ are not about saving the planet—it will always survive and evolve with new combinations of atom—but about the prosperous development of our own species.” —Carl Folke is the science director of the Stockholm Resilience Centre at Stockholm University.

“The same mathematics of networks that governs the interactions of molecules in a cell, neurons in a brain, and species in an ecosystem can be used to understand the complex interconnections between people, the emergence of group identity, and the paths along which information, norms, and behavior spread from person to person to person.” —James Fowler is a political scientist at the University of California, San Diego.

“We started human life as hunter-gatherers, where contact with others, kin and non-kin, was the center of human life, social and moral. Begin by holding hands and talking, face to face, recalling our shared evolutionary history, and the importance of human nature.” —Marc Hauser is an evolutionary biologist at Harvard University.

“The dazzling diversity of species and biological adaptations over 3.5 billion years of life on Earth owes its existence to “adaptation by natural selection,” which requires just three simple conditions to operate: variation, differential selection (the best performing traits survive and reproduce more effectively than others), and replication of successful traits by subsequent generations, via a double helix of molecules that code for proteins as biological building blocks, or among more complex animals, via imitation or cultural transmission of methods and knowledge.” —Dominic Johnson is a reader in politics and international relations at Edinburgh University.

The biosphere is the largest and most important asset of our planet—a vast living natural market that contains and makes our individual lives, human society, and the economy possible.” —Enric Sala is a National Geographic Fellow and associate professor at Spain’s National Research Council.

“Frequently, the way to understand a complicated system is to understand its component parts, but that’s probably not the case for the most interesting complicated systems—like us.” —Robert Sapolsky is a biologist and professor of neurology at Stanford University.

“You can make sense of anything that changes smoothly in space or time, no matter how wild and complicated it may appear, by reimagining it as an infinite series of infinitesimal changes, each proceeding at a constant (and hence much simpler) rate, and then adding all those simple little changes back together to reconstitute the original whole.” —Steven Strogatz is a mathematician at Cornell University.

“Many social and natural phenomena—societies, economies, ecosystems, climate systems—are complex evolving webs of interdependent parts whose collective behavior cannot be reduced to a sum of parts; small, gradual changes in any component can trigger catastrophic and potentially irreversible changes in the entire system that can propagate, in domino fashion, even across traditional disciplinary boundaries.” —George Sugihara is a theoretical biologist at the Scripps Institution of Oceanography.

“I take the easy way out by quoting another eminent scientist. In Cosmos Carl Sagan said, ‘We are made of star stuff.’ That simple statement does not encompass the physics of the earliest moments of the universe, but it encompasses its evolutionary history, from the formation of the first stars, which enriched the universe with additional elements, to the creation of planetary systems, and life and humanity on the planet Earth. Because it emphasizes our intimate and direct connection with the cosmos, it admits the possibility that others are, or have been, or may be, likewise connected.” —Jill Tarter is the director of the SETI Institute’s Center for SETI Research.

Knowledge is a public good and increases in value as the number of people possessing it increases.” —John Wilbanks is vice president of science at Creative Commons.

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Richard Feynman Teaches you the Scientific Method

The scientific method refers to a process of thought based on integrating previous knowledge, observing, measuring, and logical reasoning.

“If it disagrees with experiment, it’s wrong. In that simple statement is the key to science.”

— Richard Feynman

In this short video taken from his lectures, Physicist Richard Feynman offers perhaps one of the greatest definitions of science and the scientific method that I’ve ever heard. And he does it in about a minute.

Now I’m going to discuss how we would look for a new law. In general, we look for a new law by the following process. First, we guess it (audience laughter), no, don’t laugh, that’s the truth. Then we compute the consequences of the guess, to see what, if this is right, if this law we guess is right, to see what it would imply and then we compare the computation results to nature or we say compare to experiment or experience, compare it directly with observations to see if it works.

If it disagrees with experiment, it’s wrong. In that simple statement is the key to science. It doesn’t make any difference how beautiful your guess is, it doesn’t matter how smart you are who made the guess, or what his name is … If it disagrees with experiment, it’s wrong. That’s all there is to it.

For more color watch the longer version below, which offers the next 9 minutes of the lecture. In this clip Feynman explains that guessing is not unscientific: “It is not unscientific to take a guess, although many people who are not in science believe that it is.”

The Scientific Method is part of the Farnam Street Latticework of Mental Models.

18 Truths: The Long Fail of Complexity

The Eighteen Truths

The first few items explain that catastrophic failure only occurs when multiple components break down simultaneously:

1. Complex systems are intrinsically hazardous systems.

The frequency of hazard exposure can sometimes be changed but the processes involved in the system are themselves intrinsically and irreducibly hazardous. It is the presence of these hazards that drives the creation of defenses against hazard that characterize these systems.

2. Complex systems are heavily and successfully defended against failure.

The high consequences of failure lead over time to the construction of multiple layers of defense against failure. The effect of these measures is to provide a series of shields that normally divert operations away from accidents.

3. Catastrophe requires multiple failures – single point failures are not enough.

Overt catastrophic failure occurs when small, apparently innocuous failures join to create opportunity for a systemic accident. Each of these small failures is necessary to cause catastrophe but only the combination is sufficient to permit failure.

4. Complex systems contain changing mixtures of failures latent within them.

The complexity of these systems makes it impossible for them to run without multiple flaws being present. Because these are individually insufficient to cause failure they are regarded as minor factors during operations.

5. Complex systems run in degraded mode.

A corollary to the preceding point is that complex systems run as broken systems. The system continues to function because it contains so many redundancies and because people can make it function, despite the presence of many flaws.

Point six is important because it clearly states that the potential for failure is inherent in complex systems. For large-scale enterprise systems, the profound implications mean that system planners must accept the potential for failure and build in safeguards. Sounds obvious, but too often we ignore this reality:

6. Catastrophe is always just around the corner.

The potential for catastrophic outcome is a hallmark of complex systems. It is impossible to eliminate the potential for such catastrophic failure; the potential for such failure is always present by the system’s own nature.

Given the inherent potential for failure, the next point describes the difficulty in assigning simple blame when something goes wrong. For analytic convenience (or laziness), we may prefer to distill narrow causes for failure, but that can lead to incorrect conclusions:

7. Post-accident attribution accident to a ‘root cause’ is fundamentally wrong.

Because overt failure requires multiple faults, there is no isolated ’cause’ of an accident. There are multiple contributors to accidents. Each of these is necessary insufficient in itself to create an accident. Only jointly are these causes sufficient to create an accident.

The next group goes beyond the nature of complex systems and discusses the all-important human element in causing failure:

8. Hindsight biases post-accident assessments of human performance.

Knowledge of the outcome makes it seem that events leading to the outcome should have appeared more salient to practitioners at the time than was actually the case. Hindsight bias remains the primary obstacle to accident investigation, especially when expert human performance is involved.

9. Human operators have dual roles: as producers & as defenders against failure.

The system practitioners operate the system in order to produce its desired product and also work to forestall accidents. This dynamic quality of system operation, the balancing of demands for production against the possibility of incipient failure is unavoidable.

10. All practitioner actions are gambles.

After accidents, the overt failure often appears to have been inevitable and the practitioner’s actions as blunders or deliberate willful disregard of certain impending failure. But all practitioner actions are actually gambles, that is, acts that take place in the face of uncertain outcomes. That practitioner actions are gambles appears clear after accidents; in general, post hoc analysis regards these gambles as poor ones. But the converse: that successful outcomes are also the result of gambles; is not widely appreciated.

11. Actions at the sharp end resolve all ambiguity.

Organizations are ambiguous, often intentionally, about the relationship between production targets, efficient use of resources, economy and costs of operations, and acceptable risks of low and high consequence accidents. All ambiguity is resolved by actions of practitioners at the sharp end of the system. After an accident, practitioner actions may be regarded as ‘errors’ or ‘violations’ but these evaluations are heavily biased by hindsight and ignore the other driving forces, especially production pressure.

Starting with the nature of complex systems and then discussing the human element, the paper argues that sensitivity to preventing failure must be built in ongoing operations.

In my experience, this is true and has substantial implications for the organizational culture of project teams:

12. Human practitioners are the adaptable element of complex systems.

Practitioners and first line management actively adapt the system to maximize production and minimize accidents. These adaptations often occur on a moment by moment basis.

13. Human expertise in complex systems is constantly changing


Complex systems require substantial human expertise in their operation and management. Critical issues related to expertise arise from (1) the need to use scarce expertise as a resource for the most difficult or demanding production needs and (2) the need to develop expertise for future use.

14. Change introduces new forms of failure.

The low rate of overt accidents in reliable systems may encourage changes, especially the use of new technology, to decrease the number of low consequence but high frequency failures. These changes maybe actually create opportunities for new, low frequency but high consequence failures. Because these new, high consequence accidents occur at a low rate, multiple system changes may occur before an accident, making it hard to see the contribution of technology to the failure.

15. Views of ’cause’ limit the effectiveness of defenses against future events.

Post-accident remedies for “human error” are usually predicated on obstructing activities that can “cause” accidents. These end-of-the-chain measures do little to reduce the likelihood of further accidents.

16. Safety is a characteristic of systems and not of their components

Safety is an emergent property of systems; it does not reside in a person, device or department of an organization or system. Safety cannot be purchased or manufactured; it is not a feature that is separate from the other components of the system. The state of safety in any system is always dynamic; continuous systemic change insures that hazard and its management are constantly changing.

17. People continuously create safety.

Failure free operations are the result of activities of people who work to keep the system within the boundaries of tolerable performance. These activities are, for the most part, part of normal operations and superficially straightforward. But because system operations are never trouble free, human practitioner adaptations to changing conditions actually create safety from moment to moment.

The paper concludes with a ray of hope to those have been through the wars:

18. Failure free operations require experience with failure.

Recognizing hazard and successfully manipulating system operations to remain inside the tolerable performance boundaries requires intimate contact with failure. More robust system performance is likely to arise in systems where operators can discern the “edge of the envelope”. It also depends on providing calibration about how their actions move system performance towards or away from the edge of the envelope.