After Pythagorean theorem was put forward in the first mathematical crisis, hippasus, a member of its school, considered a question: What is the diagonal length of a square with a side length of 1? He found that this length can not be expressed by integer or fraction, but only by a new number. Hippasus's discovery led to the birth of the first irrational number √2 in the history of mathematics. The appearance of small √2 set off a huge storm in the mathematics field at that time. It directly shook the Pythagorean school's mathematical belief and made the Pythagorean school panic. In fact, this great discovery is not only a fatal blow to Pythagoras school. This was a great shock to the thoughts of all the ancient Greeks at that time. The paradox of this conclusion lies in its conflict with common sense: any quantity can be expressed as a rational number within any precision range. This is a widely accepted belief not only in Greece at that time, but also in today's highly developed measurement technology. However, the conclusion that is convinced by our experience and completely in line with common sense is overturned by the existence of a small √2! How contrary to common sense and ridiculous this should be! It just subverts the previous understanding. To make matters worse, people are powerless in the face of this absurdity. This directly led to the crisis of people's understanding at that time, which led to a big storm in the history of western mathematics, known as the "first mathematical crisis." The second mathematical crisis stems from the use of calculus tools. With the improvement of people's understanding of scientific theory and practice, calculus, a sharp mathematical tool, was discovered independently by Newton and Leibniz almost simultaneously in the seventeenth century. As soon as this tool came out, it showed its extraordinary power. After using this tool, many difficult problems have become easy. But Newton and Leibniz's calculus theory is not strict. Their theories are all based on infinitesimal analysis, but their understanding and application of the basic concept of infinitesimal is confusing. Therefore, calculus has been opposed and attacked by some people since its birth. Among them, the most violent attack was British Archbishop Becquerel. The Third Mathematical Crisis1In the second half of the 9th century, Cantor founded the famous set theory, which was severely criticized by many people as soon as it came into being. But soon this groundbreaking achievement was accepted by mathematicians and won wide and high praise. Mathematicians found that starting from natural numbers and Cantor's set theory, the whole mathematical building could be established. Therefore, set theory has become the cornerstone of modern mathematics. The discovery that "all mathematical achievements can be based on set theory" intoxicated mathematicians. 1900, at the international congress of mathematicians, poincare, a famous French mathematician, declared cheerfully: "… with the help of the concept of set theory, we can build the whole mathematical building … today, we can say that we have reached absolute strictness …"