Over 500,000 people visited Farnam Street last month to expand their knowledge and improve their thinking. Work smarter, not harder with our free weekly newsletter that's full of time-tested knowledge you can add to your mental toolbox.

Garrett Hardin on the Three Filters Needed to Think About Problems

One of the best parts of Garrett Hardin‘s wonderful Filters Against Folly is when he explores the three filters that help us interpret reality. No matter how much we'd like it to, the world does not only operate in our circle of competence. Thus we must learn ways to distinguish reality in areas where we lack even so much as a map.

Hardin's genius reminds us of this quote by Sports Illustrated's Andy Benoit.

Andy Benoit

Mental Tools

We need not be a genius in every area but we should understand the big ideas of most disciplines and try to avoid fooling ourselves. That's the core to the mental models approach. When you're not an expert in a field, often the best approach is one that avoids stupidity. There are few better ways of avoiding stupidity than understanding how the world works.

Hardin begins by outlining his goal: to understand reality and understand human nature as it really is, removing premature judgment from the analysis.

He appropriately quotes Spinoza, who laid out his principles for political science thusly:

That I might investigate the subject matter of this science with the same freedom of spirit we generally use in mathematics, I have labored carefully not to mock, lament, or execrate human actions, but to understand them; and to this end I have looked upon passions such as love, hatred, anger, envy, ambition, pity, and other perturbations of the mind, not in the light of vices of human nature, but as properties just as pertinent to it as are heat, cold, storm, thunder, and the like to the nature of the atmosphere.

The goal of these mental filters, then, is to understand reality by improving our ability to judge the statements of experts, promoters, and persuaders of all kinds. As the saying goes, we are all laymen in some field.

Hardin writes:

What follows is one man's attempt to show that there is more wisdom among the laity than is generally concluded, and that there are some rather simple methods of checking on the validity of the statements of experts.

Three Filters Needed to Think About Problems

Literate Filter

The first filter through which we must interpret reality, says Hardin, is the literate filter: What do the words really mean? The key to remember is that Language is action. Language is not just a way to communicate or interpret; language acts as a call to, or just as importantly, an inhibitor to action.

The first step is to try to understand what is really being said. What do the words and the labels actually mean? If a politician proposes a “Poverty Assistance Plan,” that sounds almost inarguably good, no? Many a pork-barrel program has passed based on such labels alone.

But when you examine the rhetoric, you must ask what those words are trying to do: Promote understanding, or inhibit it? If the program had a rational method of assistance to the deserving poor, the label might be appropriate. If it was simply a way to reward undeserving people in his or her district for their vote, the label might be simply a way to fool. The literate filter asks if we understand the true intent behind the words.

In a chapter called “The Sins of the Literate,” Hardin discusses the misuse of language by examining literate, but innumerate, concepts like “indefinite” or “infinite”:

He who introduces the words “infinity” or any of its derivatives (“forever” or “never” for instance) is also trying to escape discussion. Unfortunately he does not honestly admit the operational meaning of the high-flown language used to close off discussion. “Non-negotiable” is a dated term, no longer in common use, but “infinity” endures forever.

Like old man Proteus of Greek mythology, the wish to escape debate disguises itself under a multitude of verbal forms: infinity, non-negotiable, never, forever, irresistible, immovable, indubitable, and the recent variant “not meaningfully finite.” All these words have the effect of moving discussion out of the numerate realm, where it belongs, and into a wasteland of pure literacy, where counting and measuring are repudiated.

Later, in the final chapter, Hardin repeats:

The talent for handling words is called “eloquence.” Talent is always desirable, but the talent may have an unfair, even dangerous, advantage over those with less talent. More than a century ago Ralph Waldo Emerson said, “The curse of this country is eloquent men.” The curse can be minimized by using words themselves to point out the danger of words. One of their functions is to act as inhibitors of thought. People need to be made allergic to such thought-stoppers as infinity, sacred, and absolute. The real world is a world of quantified entities: “infinity” and its like are no words for quantities but utterances used to divert attention from quantities and limits.

It is not just innumerate exaggeration we are guarding against, but the literate tendency to replace actors with abstractions, as Hardin calls it. He uses the example of donating money to a poor country (Country X), which on its face sounds noble:

Country X, which is an abstraction, cannot act. Those who act in its name are rich and powerful people. Human nature being what it is, we can be sure that these people will not voluntarily do anything to diminish either their power or their riches…

Not uncommonly, the major part of large quantities of food sent in haste to a poor country in the tropics rot on the docks or is eaten up by rats before it can be moved to the people who need it. The wastage is seldom adequately reported back to the sending country…(remember), those who gain personally from the shipping of food to poor nations gain whether fungi, rats, or people eat the food.

The Numerate Filter

Hardin is clear on his approach to numerical fluency: The ability to count, weigh, and compare values in a general or specific way is essential to understanding the claims of experts or assessing any problem rationally:

The numerate temperament is one that habitually looks for approximate dimensions, ratios, proportions, and rates of change in trying to grasp what is going on in the wold. Given effective education–a rare commodity, of course–a numerate orientation is probably within the reach of most people.

Just as “literacy” is used here to mean more than merely reading and writing, so also will “numeracy” be used to mean more than measuring and counting. Examination of the origins of the sciences shows that many major discoveries were made with very little measuring and counting. The attitude science requires of its practitioners is respect, bordering on reverence, for ration, proportions, and rates of change.

Rough and ready back-of-the-envelope calculations are often sufficient to reveal the outline of a new and important scientific discovery….In truth, the essence of many of the major insights of science can be grasped with no more than child's ability to measure, count, and calculate.


To explain the use of the literate and numerate filters together, Hardin uses the example of the Delaney Amendment, passed in 1958 to restrict food additives. This example should be familiar to us today:

Concerned with the growing evidence that many otherwise useful substances can cause cancer, Congress degreed that henceforth, whenever a chemical at any concentration was found to cause cancer–in any fraction of any species of animal–that substance must be totally banned as an additive to human food.

From a literate standpoint, this sounds logical. The Amendment sought to eradicate harmful food additives that the free market had allowed to surface. However, says Hardin:

The Delaney Amendment is a monument to innumerate thought. “Safe” and “unsafe” are literate distinctions; nature is numerate. Everything is dangerous at some level. Even molecular oxygen, essential to human life, becomes lethal as the concentration approaches 100 percent.

Sensitivity is ordinarily expressed as “1 part per X,” where X is a large number. If a substance probably increases the incidence of cancer at a concentration of 1 part per 10,000, one should probably ban it at that concentration in food, and perhaps at 1 in 100,000. But what about 1 part per million?…In theory there is no final limit to sensitivity. What about 1 milligram per tank car? Or 1 milligram per terrestrial globe?

Obviously, some numerical limits must be applied. This is the usefulness of the numerate filter. As Charlie Munger says, “Quantify, always quantify.”


Hardin introduces his final filter by requiring that we ask the question “And then what?”  There is perhaps no better question to prompt second-order thinking.

Even if we understand what is truly being said and have quantified the effects of a proposed policy or solution, it is imperative that we consider the second layer of effects or beyond. Hardin recognizes that this opens the door for potentially unlimited paralysis (the poorly understood and innumerate “Butterfly Effect”), which he boxes in by introducing his own version of the First Law of Ecology:

We can never merely do one thing.

This is to say, all proposed solutions and interventions will have a multitude of effects, and we must try our best to consider them in their totality.

In proposing this filter, Hardin is very careful to guard against the Slippery Slope argument, or the idea that one step in the wrong direction will lead us directly to Hell. This, he says, is a purely literate but wholly innumerate approach to thinking.

Those who take the wedge (Slippery Slope) argument with the utmost seriousness act as though they think human beings are completely devoid of practical judgment. Countless examples from everyday life show the pessimists are wrong…If we took the wedge argument seriously, we would pass a law forbidding all vehicles to travel at any speed greater than zero. That would be an easy way out of the moral problem. But we pass no such law.

In reality, the ecolate filter helps us understand the layers of unintended consequences. Take inflation:

The consequences of hyperinflation beautifully illustrate the meaning of the First Law of Ecology. A government that is unwilling or unable to stop the escalation of inflation does more than merely change the price of things; it turns loose a cascade of consequences the effects of which reach far into the future.

Prudent citizens who have saved their money in bank accounts and government bonds are ruined. In times of inflation people spend wildly with little care for value, because the choice and price of an object are less important than that one put his money into material things. Fatalism takes over as society sinks down into a culture of poverty….

To Conclude

In the end, the filters must be used wisely together. They are ways to understand reality, and cannot be divorced from one another. Hardin's general approach to thinking sums up much like his multi-disciplinary friend Munger's:

No single filter is sufficient for reaching a reliable decision, so invidious comparisons between the three is not called for. The well-educated person uses all of them.

Check out our prior posts about Filters Against Folly: