The cow on the open land is an important thought experiment in the field of cognitive theory. It describes a peasant who feared that the cows he won would be lost. At this time the milkman arrived at the farm. He told the peasant not to worry because he saw the cow in an open space nearby. Although the peasant believed in the milk worker, he still looked at the cow personally. He was satisfied by seeing the familiar black and white shapes. After a while, the milkman was reconfirmed in the open space. The cow was indeed there, but it was hiding in the woods and there was a large piece of black and white paper wrapped around the tree in the open space. It was clear that the farmer had mistaken this paper as his cow. The problem arose. Although the cow were always on the open land, but were it correct when the farmer said that he knew the cow on the open land?
    The cow paradox were originally used by Edmund Gettier to criticize the JTB (justified true belief) theory as the definition of knowledge in the mainstream, that is, when people believe in one thing, it becomes knowledge; this matter is true in fact, and people have reason to believe it. Western philosophers have been confused about the standard of truth, and their answers to Gettier proves it particularly. Jonthan Dancy said, "Of the countless papers written to answer Gettier, many gave the impression that answering Gettier was a privat philosophical game, meaningless except to the player."([America]J.Dancy. An Introduction to Contemporary Epistemology.Translated by Wenzhang Zhou, Baogang He. Beijing: Remin University of China Press, 1990,pp.29)  (Wenzhang Zhou said in his "Posttranslation notes" that Dansy's "An Introduction to Contemporary Epistemology" has been widely used by many universities in Britain and the United States.) Dancy himself argued at length and could not make it clear, either.

    In this experiment, the peasant believed that the cow was on the ground and were confirmed by the testimony of the milkman and his observation of the black and white objects on the open ground, and later confirmed by the milkman, this is true. Even so, the peasants did not really know where the cow was because his knowledge of cow there was based on the wrong premise. Gettier used this experiment and some other examples to explain that the theory of defining knowledge as JTB needs to be revised.

The Gettier problem belongs to the skepticism, and the history of skepticism can be traced back to Descartes and even earlier. Black and white paper is an illusion, then how much of our knowledge is truth? Descartes after careful thought found that all our knowledge may be illusion.

The more learned a person is, the more he or she will be careful not to be too convinced. Any accident or miracle may happen. People's cognitive ability is too limited. There are already too many things that made us staggering. Any judgment that we think we have absolute certainty can be denied. The Newtonian mechanics we now believe may be an illusion. Relativity theory may be an illusion. Scientific knowledge of quantum mechanics, geometry, chemistry, biology, sociology, and so on may be false impressions. All axioms and logic systems may be wrong. Especially what we call objective laws may be illusions. Kant proposed a Copernican revolution and believed that all so-called objective knowledge must be subjective.

We can believe that mankind has mastered many truths, but in the end it is impossible to judge what is truth. When it comes to any specific theory, we have no good reason to prove that it is objective truth. So we can only say that there is no absolute truth in the world. Descartes made us doubt everything, and Carl Marx's motto is also to doubt everything.

Popper pointed out that the conclusions obtained by incomplete induction are impossible to be confirmed. For example, all crows in the world are black is an inductive conclusion. After observing 10,000 black crows, there is no guarantee that the next crow is not white. All the human exceptional functions we saw are deceptive, but we cannot theoretically prove that there is no exceptional function in the world. Gödel's Incompleteness Theorem tells us that the biggest major premise of the deductive method cannot come from deduction but only from induction. Therefore, all our knowledge from deductive method is ultimately unprovable, and all science is built on the beach.

If our knowledge is illusion, then does scientific research have meaning? Yes. Scientific meaning does not come from objectivity, but from usefulness or practica- lity.

I am hungry now, I want to eat crabs. I want to know if crabs are poisonous. Popper told us: No matter what method you use, you can not prove crabs are non-toxic? Even if you have taken 10 bites, you can't guarantee that the 11th bite is non-toxic. You can not even prove rice non-toxic. So what should you do? Starve to death? This misunderstanding turned Popper's theory into a fasting theory. The responsibility of scientists is not to be miserable, not to sigh, and it is the responsibility of scientists to find a way out for humanity. Although Popper's conclusion is of great theoretical significance, what people need is more of a proof theory. If there is no absolutely confirmed theory, at least we need a relatively proven theory, although the relatively proven theory may cause poisoning, but people also willing to accept, because we do not want to starve to death.

Humans have been using relatively proven theories since ancient times. These theories will have a variety of errors, but they have great practical significance.They can only be perfected gradually and cannot be abandoned. There was hardly any truth in the earliest human hands. There were only some analogical revelations, most of which were wrong. Therefore, human survival ability at that time was very low. But it was such revelations that enabled them to survive and gradually grew stronger. The mistake of Dancy and many western scholars is that they did not consider the question of truth standard from the point of view of practice.

When I said theory is illusion, I did not mean that scientists deliberately falsified. When I said the value of theory comes from practicality, I did not mean instant benefits. The practicality here refers to the promotion of people's interests, including both self-interests and the interests of others, both immediate and long-term interests.