French mathematician Émile Borel was an exceptionally versatile and productive scientist. He left behind a vast scientific legacy consisting of numerous original works, monographs and textbooks relating to various fields of mathematics and its applications to physics and other branches of natural science. The book shows those stages through which the concept of probability passes - first opposed to the concept of reliability, then merging with what can be called practical reliability, and then with reliability, which could be characterized as absolute if it were necessary to clarify its meaning.