, 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.