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