# How do people use logic

## Logic and intuitionThe other side of reason

The fifth century BC: At the beginning there is the "nous". With it, explains Parmenides, man recognizes the essence of being.

• Premise 1: All people are mortal.
• Premise 2: Parmenides is human.

The third century before Christ. Aristotle developed logic as an independent system of correct inferences.

• Conclusion: Parmenides is mortal.

The 19th century. Logic is mathematized and expanded into a comprehensive concept of formal thinking. One begins to equate rational thinking and logic. Intuition: degraded to a minor matter.

The presence. We don't follow the rules of logic, probability theory, mathematical decision theory and so on. Are we returning to a more holistic view of human thought? Markus Knauff:

"And the question is, how can we explain this, why are we doing this?"

• When it rains, the road is wet.
• Ergo: It rained.

The road is wet, so it must have rained. Or? (picture alliance / dpa / Robert B. Fishman, ecomedia)

It sounds kind of logical, but of course it's wrong. Because if-then statements cannot be easily reversed. After all, it could be that the street is wet because the street cleaning service was on the way. But - it cannot be denied: people make such mistakes, and not so rarely. Next example:

• When people think logically, they are rational.
• People who don't think logically are not rational.

### "You should actually go by the Speak rationalities "

Anyone who thinks logically correct will surely meet an important criterion for rationality. But does that also mean that someone is irrational if he does not follow the iron rules of logic?

"I am very convinced that there is absolutely no one norm with which human thinking can be described or by which human thinking must be measured. I believe that one should not really be the Speaking of rationality, but of the Rationalities. "

Markus Knauff, professor of psychology at the University of Giessen, wants to overcome the old dogmas that tie rationality too strongly to philosophical logic:

"So far it has been the case that philosophy has defined what we want to see as rational and psychology has then oriented itself towards this target value and examined where people deviate from these norms. And the idea of ​​the priority program is to combine psychology and philosophy Because the observation is that people, even if they deviate from some norms that come from philosophy, get along well in life and also make sensible decisions. "

Since 2011, a priority program of the German Research Foundation has tried to bring two questions together. Knauff is the speaker:

"The first question is 'How should we think?' And the second question is 'How do we really think?' "

• All French are wine drinkers.
• Some wine drinkers are gourmets.
• So: some French are gourmets.

"A typical example that the logic teacher also grapples with in his courses. We know the premises are true, we know the conclusion is true, so the conclusion seems plausible to us."

Like all his compatriots, he drinks wine, and on top of that he's still a gourmet: a Frenchman, logically, in a gourmet restaurant (picture-alliance / dpa / Udo Bernhart)

Gerhard Schurz, Professor of Theoretical Philosophy at the University of Düsseldorf. A specialist in logic, evolution and science theory.

"And now you have to make it clear, but in the logical sense it is not valid, because it could be that the premises are true, that in fact the French are all wine drinkers, but understand nothing about gourmet and delicate food preparation. That is not the case in our world but in a logically possible world and the logical validity should apply not only in our factual environment, but in all possible environments. This is the characteristic of deductive logic that the conclusion with certainty, i.e. in all possible logical worlds This distinction is not always important for the practical person, most of the time I would even say 'not important' and therefore it is not made in everyday life. "

### Rational behavior consists of three different ways of thinking

Why bother to stick to abstract rules when you just know that some French are gourmets? A look into the brain also shows that thought processes in terms of content are by no means subordinate to logical thinking, but can work independently of them. Markus Knauff:

"They can also show that in patients who have injuries to certain brain structures from tumors, strokes, that they are either impaired in logical abstract reasoning or cannot benefit from the fact that the content is plausible. People without these brain lesions benefit from something plausible, these patients are not. And you can also use a method called transcranial brain stimulation to produce short-term lesions in the brain by producing magnetic fields. Here, too, you can show that you can do content-related tasks and abstract logical tasks or even probability-based tasks can interfere with these differentially. So there is some evidence that there are different processing networks for abstract and concrete conclusions. "

When it comes to rational behavior, three different players seem to be in the running: logical thinking, thinking in terms of statistical probabilities and concrete thinking that is based on plausible content. Conclusions based on the latter.

• The road is wet so it rained.

They may or may not agree with the logic and norms of probability. And they can be based on proven knowledge or on intuitive assumptions. How can the value of these three ways of thinking be judged? Gerhard Schurz considers them in the context of the theory of evolution:

"The process of evolution selects out a lot."

Logical reasoning apparently emerged early in evolution. For example in primates.

• Assumption 1: A beam balance leans when a weight is placed on one of its sides.
• Assumption 2: bananas have weight.
• Conclusion: If you put a banana in one of the scales, it has to be where the scales are tilted.

Chimpanzees drew this conclusion in an experiment without being able to see the banana.

Knows how to eat a balanced diet: Chimpanzees can use logic to determine which weighing pan the banana is in (dpa picture alliance / Julian Stratenschulte)

These logical systems were refined more and more and were reflected in linguistic arguments in humans. Gerhard Schurz, like most evolution theorists at present, assumes that it happened for social motives:

"Imagine, in the old Stone Age, a group of maybe up to 100 people who hunt together, who share their food, that is, their hunted prey, then it has to be according to social rules and it turns out that if it is about the uncovering of rule-breaking, people suddenly master certain logical conclusions perfectly, which they do not master in other contexts, but constantly make mistakes. "

### Abstract logic versus logic in social situations

The famous Wason card test. Don't even try to pass it.

The following rule should apply: If there is an "A" on the front of a card, then there is a "1" on the back. There are four cards in front of them. With two of them you can see the front, with the other two the back. There is an "A" on the front of one card and a "B" on the second. There is a "one" on the back of the third card and a "two" on the back of the fourth. Which card do you have to turn over to check if the rule applies? Apron:

"The real answer of logic is: you have to turn over the card with the A and the side with the two - nobody can do that, nobody takes the card with the two. But then exactly the same attempt was made with the following rule: It's about Young people in a bar where cola and beer are served and the rule is: whoever drinks alcohol, i.e. drinks beer, must be at least 16 years old. "

Four young people are sitting in the bar. One drinks beer, another coke. You don't see what the others are drinking, but you know that one is eighteen and the other fourteen. With which young people do they have to check whether the rule is being observed? Are you just asking the beer drinker how old he is or are you also investigating another young person? Here again the rule that needs to be checked: "Anyone who drinks beer must be at least sixteen years old."

Gerhard Schurz: "Everyone then looks at the 14-year-old - is mastered perfectly in this example and in the other example, although it is logically exactly the same task, at most two or three percent of all test subjects can do it."

In vivid situations involving social conventions, almost all people can draw certain logical conclusions which they fail when formulated in an abstract way. Logical thinking seems to have been rewarded in evolution because it helps to cope with social situations. The ability to judge something as true or false provides a survival advantage. In addition, evolution has selected completely different phenomena, explains Gerhard Schurz:

"Of course, there are also evolutionary effects of our belief system that are independent of the truth value. I have called this the generalized placebo effect. I'll give you an example. If you believe that the disease you are suffering from is yours is surmountable, and will soon be gone, then they will try to overcome the disease in good spirits and cheerful, regardless of whether that is really the case or not. That is, the mere belief in a positive event gives a certain strength, a certain self-confidence and that's what I call the generalized placebo effect. "

Placebo effect: Belief in a positive result gives strength. (imago)

### People tend to rash generalizations

Studies show: The placebo effect has engraved itself deeply in our thinking. Psychologists also speak of "inductive overconfidence", for example, of the exaggerated confidence in being able to find a rule quickly. Gerhard Schurz:

"Inductive overconfidence in the sense that people are inclined to rash generalizations even from very small samples. This inductive overconfidence naturally also has its advantages when one has to close very quickly, so one is forced when one decides whether to If you want to continue hiking in or that direction or to go where there are probably better food sources, food resources, then you have to decide very quickly, of course the inductive overconfidence can also lead to false conclusions and then lead above all in the social area, yes, that people are quickly judged, given a rubbish brush, inferred from the outside to the inside, so this whole prejudice formation is of course also connected with inductive overconvidence. "

Of course, such quick decisions are more prone to error and more risky than carefully thought-out decisions. But sometimes they are just inevitable, says Markus Knauff:

"So if you ask yourself why people make mistakes at all, then it is that we basically have the competence to think logically correctly and correctly with regard to probability, but that there are a lot of limitations, limitations of our cognitive system. We do not have unlimited cognitive resources; not all information is available. "

The working memory of the brain can only store and process a limited amount of information, whereas the variety of information from the outside world is in principle unlimited. Therefore, according to Magnus Knauff, we also have to include the so-called heuristics in the realm of rationality, which have been extensively researched in recent years.

These are rules of thumb that are used intuitively when something can no longer be mastered with the mind alone. One then uses, for example, an old thought pattern that has already proven itself. Or focus on one or two prominent signposts of a situation and neglect the others. Seen in this way, rationality encompasses all forms of thinking that promise the most efficient solution to a problem. Markus Knauff:

"I believe that the environment makes different demands on what we want to see as rational. In some situations we have to choose between two alternatives, then classical logic really helps us. In some cases we also have to decide whether something is more likely In some cases, if we are satisfied with a good but not necessarily the best solution, all of this influences, I believe, which norms we have to use in order to make a decision what we want to consider rational. "

• The goal lies behind the mountain.
• One of two paths leads to the goal, the other leads astray.
• One way is narrow but stable.
• The other way is wide but full of holes.
• Question: Which way leads to the goal?

### "My results sound pretty paradoxical - especially for experts"

In the long term, the rationality researchers want to define more precisely when which type of thinking is to be used optimally - and that is to say in a rational way. A fundamental question arises here. On the one hand, people can think through problems logically or in terms of their probabilities. On the other hand, make quick decisions intuitively. But how are both related? How is it regulated which thinking comes into play when?

"For a lot of people, especially for the experts, my results sound pretty paradoxical."

The psychologist Wim de Neys from the National Center for Scientific Research in Paris has caused a stir in recent years. Because his studies suggest that there is not only the well-known intuition with which we decide spontaneously and automatically from the gut, but also a logical intuition. Wim de Neys gave test subjects tasks in which they intuitively quickly come across a solution - which is wrong.

• The racket-ball problem:
• A racket and a ball together cost one euro and ten cents.
• The racket costs one euro more than the ball.
• How much does the ball cost?

Most test subjects immediately say "The ball costs ten cents" and are intuitively completely sure. But the answer is wrong. If the ball cost ten cents and the club is one euro more expensive, it would have to cost one euro and ten cents. One euro and ten cents for the club plus ten cents for the ball result in one euro and twenty cents.

Almost all test subjects who initially give the intuitively wrong answer can easily see the correct solution if they think about it for a moment: five cents for the ball, one euro and 5 cents for the bat. But why do they give a wrong answer beforehand at all? Psychologists usually explain this with the fact that the test subjects perceive the relative statement "The racket is one euro more expensive than the ball" as an absolute statement: "The racket costs one euro". Then the quick, automatic answer would be correct.

Today's special offer: rackets and balls are available as a logic experiment for just one euro and ten cents (imago / Schwörer)

### There is apparently a "logical intuition"

Wim de Neys wanted to know exactly. And asked 13 test subjects into the brain scanner:

"We know that a region in the middle of the frontal lobe is responsible for monitoring conflicts or errors, and we examined its role. So we gave the test subjects tasks such as the ball-racket problem, in which a conflict between the intuitive and intuitive ones through relative formulations and the logically correct solution occurs. "The racket is one euro more expensive than the ball" compared to easily solvable tasks in which this conflict does not occur. There there were absolute formulations such as: "The racket costs one euro". Observed in the brain scanner In addition to the conflict monitoring region, we also have the anterior right frontal lobe region, which is involved when a process in the brain is inhibited. "

"The main finding, however, was that the conflict monitoring region in the brain was always active in the test subjects, regardless of whether they gave the intuitively incorrect or the logically correct answer.Hence we conclude that people are somehow subconsciously aware of the conflict even if they give the wrong intuitive answer. "

This is the paradoxical result of the studies by Wim de Neys, about which the professional world is so amazed. There seems to be a "logical intuition" and it manifests itself in an automatically generated signal in the brain. This signal remains unconscious, but is also expressed physically:

"The conflict monitoring region also regulates the autonomic nervous system. Whenever a conflict is discovered, the autonomous nervous system is also briefly activated. Then you sweat a little more, for example, the body temperature changes and with it the electrical conductivity of the skin."

The brain creates a feeling that many people know when they have to crack hard logical nuts, a "gut feeling": "I have a solution, it is definitely wrong."

For Wim de Neys, these findings can only be explained as follows: Logical and intuitive thinking are closely linked in the brain:

"There are both heuristic, that is, fast and automatic intuitions and there is a kind of logical intuition that is unconsciously present in the brain as a yardstick. According to my theory, both are part of a common system. The heuristic and logical intuition become parallel and simultaneously in the brain activated. If a conflict arises between logic and intuition, the conflict signal enables a process to be initiated in which one can think more about the spontaneous, intuitive response. "

Wim de Neys was able to substantiate his model in several studies. A plausible model, because it combines the old theory, according to which logical thinking is the highest measure of rationality, with the new trend of increasing the value of intuitive thinking. Wim de Neys also assumes that a logical standard is firmly anchored in the head. But it works unconsciously and is by no means omnipotent, so it cannot completely prevent intuitive solutions. Those who think logically and intuitively survive best.

### Mental health affects logical reasoning

Markus Knauff considers Wim de Neys' findings to be extremely important because they explain how the mind can react flexibly. And it even goes one step further.

• Premise 1: There are spiders in old houses.
• Premise 2: This is an old house.
• Question: Are there spiders there?

Markus Knauff: "The effect was that phobics perform worse than people who have no arachnophobia in such conclusions. They made more logical errors and it took them longer to draw conclusions."

Spiders are - logically - always to be found in old houses (picture alliance / dpa / Romain Fellens)

• Premise 1: If nobody likes me, then my life is pointless.
• Premise 2: Nobody likes me.
• Question: Does your life still make sense?

Markus Knauff: "The second group of test persons were depressive patients and they were better at such conclusions than non-depressive patients. It is not a contradiction. Because phobics show this avoidance behavior and therefore the conclusion is more difficult for them, while depressives especially in negative thinking styles are experts right now and that is why they can do it even better. "

Knauff's thesis: The mental state influences the performance in logical reasoning.

"I believe that such findings and many others show that we do not get so much further in psychology with the consideration of isolated individual phenomena, tiny experimental findings, but that we somehow have to come up with a theory of the overall psychological process and of course there are different types of this from rationality, then our thinking can be influenced by emotions, and so on. "

A broader understanding of rationality also sharpens the view of science, which is commonly regarded as the crown of rationality. Because there are also conventions and rules of thumb in addition to pure logic.

Scientific knowledge is known to be based on experiment and exact methodology. This gives them a reputation for being particularly reliable. In principle correct, says Professor Torsten Wilholt from the University of Hanover. But there is also a problem there:

"Because now a peculiarity of empirical research comes into play, which makes empirical science so exciting and also philosophically interesting, which therefore also ensures that philosophy of science is much more exciting than logic, I now say with a slight provocation to my colleagues Colleagues: Science is based on what we call induction, that is, we start from individual observations and individual experiments and the results, but in the natural sciences in particular we always try to generalize from them. The conclusions we draw are always reflected in the content beyond what we have actually observed technically closely or proven experimentally. "

### Scientific standards cannot be derived from a higher rationality

Statistical procedures also reflect common conventions (dpa / picture alliance / University of Jena)

Each experiment has a specific design. You make hypotheses, choose laboratory animals. The result is therefore initially only valid in the context of this design. It is therefore not surprising that the results are not confirmed under different circumstances. Only when many similar experiments come to the same result can one speak of scientific knowledge. Science is a social process. It is based on scientific standards with which experiments can be compared. Such a standard is, for example, the so-called significance value p.

• The p-value indicates whether a result could have arisen purely by chance.
• The spectrum of the p-value: between zero and one.
• The higher the value, the less the result of an experiment is based on chance.

For Torsten Wilholt, the problem is that scientists often misunderstand statistical parameters such as the p-value as guarantors of absolute rationality:

"The current practices of significance testing in statistics also reflect certain conventions and in particular the so-called p-values, which stand for a statistically significant result, are nothing more than certain conventions."

The standards of science cannot be derived from a higher rationality. They too need to be defined. Is it just about basic research on cells or clinical research on humans? Should the toxicity of a substance be assessed or the age of a rock? Wilholt:

"How much empirical evidence is actually enough evidence? How good does the evidence have to be? Logic has no answer to this question, the science of statistics has no answer, and ultimately philosophy cannot answer this question either, because the question hangs starting with an assessment of the question: How bad would it be to be wrong? And how important is it to us to be right? We have to weigh that against each other and only if we can make such a weighing can we answer: how much evidence are we enough? "

Scientists drive scientific progress. The value of their findings, however, is only revealed within the framework of what the community of scientists sets as a standard and accepts as a result.

• Step 1: logic is part of rationality.
• Step 2: Intuition is part of rationality.
• Conclusion: Intuition and logic form a system called rationality.

Next step: The rationality of the individuals is part of the rationality of the collective. Markus Knauff:

"It is very important that up to now the rationality has mainly focused on the individual - how do individuals come to rational decisions? - but I also see a research perspective that we also have to consider: Is that just adding up the rationality of individuals or is it?" something like collective, common rationality is something else than just adding up individuals. "

Speakers: Claudia Matschula, Nikolaus Bender and Franz Lake
Sound and technology: Hanna Steger and Oliver Dannert
Director: Claudia Kattanek
Editor: Christiane Knoll
Online: Felix von Massenbach

Production: Deutschlandfunk 2016